Rubbing back coats of paint?

[woden]

I'm putting a plywood shelf in a boat at the moment and I'm in the process of painting it. What I'm wondering is if paintwork is the same as using varnish in that you should rub back the primer before you apply undercoat, and then rub back the undercoat before the top coat (in this case gloss)? Just as if they were various coats of varnish.

What grade of sandpaper is best for this rubbing back and should you rub back the top coat/gloss if you're applying more than one?

Also, what exactly is primer for? I've always put primer on bare wood but this is due to being told it was necessary and not because I understood the need for it. I've heard various reasons, from filling in the grain, to sealing the wood, to giving later coats a better grip and so on. But what's the real reason?

Finally, how many coats of primer/undercoat/gloss would you recommend and should the first coat of primer be thinned?

[durwood]

Varnishing is usually a description of a clear finish instead of a coloured one. Colour coats of the same family are the clear with pigment added to it.

Primer is the first coat - (from prime No 1) its main purpose is to provide adhesion for the following coats. In some cases thats all it does in others it also helps seal off the surface to prevent further coats soaking in.

Undercoat is applied to build up a surface to make it smooth/level for the top coats. Sometimes its necessary to also apply filler which is usually just thick undercoat (which fills better/dries quicker) Sometimes primers
and undercoats are combined as one but they do such different jobs its necessary to keep them as separate tins for the best results.

Rubbing back is used to do two things smooth and level the surface of what you have, be it wood /primer /undercoat/or top coat and to increase the adhesion of the next coat as paint sticks better to a surface with scratches in it than a glossy surface.

Its not always necessary to rub back as some paints are softened by the solvent of the following coat and melt into them but rubbing usually doesn't hurt anyway (it should make the surface smoother) if you want it smoother.

What grade paper- the finest you can use which will give you the result you want in a reasonable time. Which means too coarse and all you do is put deep scratches which the next coat has to fill and you run the risk of rubbing through. Too fine and you can take forever to rub the marks out and the surface ends up worse next coat anyway as your paint isn't going on as smooth as you have rubbed it.

If you keep above 200 for primers and above 800 for colours you will get good results but you can use any grade if its what you require it to do.
(Rub quickly, make it super smooth)

How many coats depends on the paint you are applying.

Enamels- 1 primer -2 undercoat - 2 colour/varnish ( if its not a great finish you didn't smooth the wood use enough filler/ undercoats. Normally the tins have instruction as to how many coats the manufacturer deems the correct amount. If you go more you must extend the drying time to make sure all appled coats are properly dry. If you brush on the paint you can only put on one coat and let it dry, if you spray its easy to keep pouring it on.

More than two coats at once of enamel will probably end in disaster as the enamel is so high a build it will wrinkle up as it dries. So its two coats let it dry for a month or so and then rub back and try again. You can add a hardener to enamel which will lock it up hard dry in 24 hours but you have just made it a 2 pack and its now become more dangerous to apply.

Lacquers are completely different, I don't think what you are doing involves them but if it does let me know.

The better you smooth the wood, the better you rub the coats of paint the better chance the final coat will come out perfect. It will never come out better than the preparation, in other words you cant expect the colour to fill the faults you leave in the preceeding coats

[woden]

Thanks for the detailed response durwood. You say that paint sticks to a surface with scratches better than one that's glossy. However, aside from shiny glosses, is this the case for undercoats and primers? These, especially the primers, always seem to dry with a dull 'roughish' sort of surface and this has always confused me about the rubbing back thing. I mean I can understand rubbing glosses and varnishes as they're shiny smooth but do the duller finishes of undercoats and primers really need it?

You say above 200 for primers and above 800 for undercoats. In that case would rubbing only with 400 and 1200 suffice for each respectively? Or should I use say 220 and 800 before going on to the 400 and 1200?

So is enamel the proper name for paint systems (primer/undercoat/colour) that result in either a gloss or satin finish? Or is it just the name for a final gloss top coat?

You also mention 2 colour/varnish. Am I right in thinking that this means you can apply varnish over the final topcoat, and if so why would you do this? Does it give a better finish or something?

So if you want to go for more than two coats of enamel (gloss?) you'd have to leave a month between the second and third coat? You mention rubbing back the second coat to apply the third but do you also rub back the first for the second?

Sorry about all the questions but I'm very much a novice. An inquisitive one at that! :)

[woden]

Another question.:rolleyes:

Would putting on a second coat of primer be a bad idea? The thing is I thinned the first one slightly thinking it might be absorbed more and stick to the bare wood better.

[durwood]

You did the right thing thinning the primer to soak into the wood. that makes it adhere better. If its too thick it doesn't penetrate. The primer must always be brushed You can spray the rest if you want.

OK lets make some paint from scratch. In this case an enamel as its the preffered finish for timber especially for a boat.

First we need to pick what sort of enamel. Its just a term for paints which have certain properties. I will keep it oversimplified.
*Dry slow
*dry glossy
*need few coats.
*needs little thinner to thin.
They are the main ones.

If we make just a simple marine enamel, we would pick a suitable resin. This is a clear substance which you would get from a manufacturer in a dry state like gravel (small chunks)

You would then dissolve it in its solvent (say turps) to make a thick varnish like substance.

To make primer add some colour (usually pink- this is just tradition as originally painters primer was made using red and white lead) now its pigment. Some other material to help it along - fillers like talc plus other to help it dry quicker or what ever. In a tin with instructions to thin more to use. It will take 24 hours to be dry.

Next I make undercoat. This time I want to make the material thick so I can fill and level the surface. So I get the clear liquid and I thicken it with powders and any other chemical it need to dry etc. It will probable be an off white colour which is Ok as it helps especially if I paint white colour over it.

Because it has been thickened it fills great, still takes 24 hours to dry and If I have started with a smooth timber surface 2-3 coats will be heaps to level the surface. It drys flat because there is so much powder in it but I don't need gloss I'm going to put over it a colour coat to do that.

I could put a second primer on coat but it won't fill as well and it will take just as long to dry so its a waste of time doing so but if done no great problem.

I could have rubbed the primer but not much is on there. The undercoat will stick as I have made my mixture to help it do this. Once I have sufficient undercoat I need to rub it down to get rid of any faults, such as:
*brush marks
*bumps in the surface
*dirt or nibs in the undercoat.
*anything else which you consider no good.

What I would like is a nice flat smooth surface for the colour . If the abrasive paper is too coarse it will leave scratches in the surface so I need to rub with paper which leaves scratches small enough for the colour to fill.

eg; if the scratches were 1mm deep the colour coat would have to be able to fill 1mm if it won't you can see the scratches as coating the surface puts an even coat on the high spots and in the scratches.

You don't need to go past 600 for enamel 320 is fine enough (it fills well.)

Now for the colour, mix the original enamel liquid with pigment and I have enamel colour. No pigment I have clear.

As I am painting undercoat I would brush one coat, let it dry still 24 hours, if I get a problem brush mark etc I can rub it out. Now the colour is glossy and it may be like a mirror, If I don't have any faults I can apply the second coat. If I spray I can apply a second coat after about 15 minutes.

*One coat usually is not enough protection and will be easy to wear through in use so hence the second coat. Again flattening will aid adhesion but it will stick perfectly well on the gloss. (enamels stick like glue)

Do a second coat and if everything goes all right you are finished. Mess up and you have to let it dry and rub it down and try again. Or wash it off straight away) How this colour coat is applied depends on the outcome, the paint is designed to produce a mirror finish off the gun or brush, so you must have all the bases covered

*smooth surface to paint on
*paint thinned properly
*the correct gun or brush
*not too much or too little paint

Do it properly and the paint flows out flat, level and glossy.

Do it wrong and the gloss or lack of shows up the faults.

You have reached the limit the manufacturer intended the paint to be subject too. If you add extra primer/undercoats/colour coats you will be reducing the durability of the paint . The usual result of too much paint is cracking.

The colour and the clear are the same material, If you wanted a clear finish you would just put it on the rubbed down timber ( 2-3 coat) If it was stain varnish it would be clear with a dye to give it colour but be transparent enough to see the wood grain.

Varnish is an old type of vegetable based clear, its used as a general name by most people for slow dry clear finish. You could put clear over a colour but it doesn't make it any better it actually takes the edge off the finish usually light colours discolour as the clear has a yellow tinge.

If the clear or colour is satin/flat/semi gloss it has had added a chemical to reduce the gloss the more you add the less gloss you get. Its still basically the same.


All the other types of paints are made up in a similar fashon as what has been described but there are many different ones. You should always pick the finish you want and then gather the primers and undercoats and any other material the manufacturer says you need to finish the process.

Paints that don't dry glossy (lacquer ) need to be buffed to get the final lustre.

Some enamels (poly) can be improved by rubbing, compounding and polishing but some can't - air dry enamel is one.

[woden]

Thanks again for the response, durwood, you're a mine of information. That post really clears up a lot of my misunderstandings.

Another question about a gloss topcoat - can gloss be thinned to make it easier to apply or would thinning remove/decrease the gloss finish?

Everything went reasonably well until I applied the gloss coat. The stuff was so thick it was very difficult to brush on smoothly. It has, however, dried better than I expected but had the stuff been somewhat thinner then I think it would have spread more consistently.

[durwood]

Hi Woden,

Yep! the thinner the material the better it flows out. What the liquid need to do is flow out flat, the thinner it is the flatter the finish the better the gloss.

thats why you need to make sure the surface is flat before you apply the final finish.

The amount of thinner needed depends on what the material is and how cold it is. The hotter the temp the thinner the material is to start with.

Usually a tin will give rough instructions as to thinning, enamel is usually about 10% but add a small amount try it on something to test it. You must use the correct thinner usually turps.

A good surface to test with is a bit of melenine chip board, rub the gloss off and wipe it clean brush the paint and wait for a few minutes you will see how well it flows out. A good quality brush is a must (nice fine bristle)

Obviously if you overdo the thinning the paint will run.

If you get a really good finish from a product and you want to repeat it measure the viscosity of the material.

To do this get a small tin (about baby food size) drill a hole (about 3mm in the middle of the bottom, try to make sure paint will flow to this hole by pushing the centre of the can down. Either completely fill the tin with your good mixture or make a permanent mark in the tin to pour it to.

Put your finger over the hole fill to the level mark look at your watch and as you take your finger away from the hole not the second hand position
(waiting till its at 12 oclock is best) let the container drain till the stream of paint stops and then starts to drip. Obviously let it drain back into the original container. Note the time when the stream stops. If you mix up more paint and thin it until you can get it to run through in exactly the same time you will have the same great mix you got before.

You can buy a proper Viscosity cup in plastic if you need to use one of these a lot but it really isn't necessary. Or maybe you can turn one on a lathe, it just needs to be a cylinder with a funnel shaped bottom and a hole in the center.

[woden]

So if you thin gloss by about 10% it won't have a negative effect on the glossiness of the finish? As I live in Ireland it's nearly always cool at best in comparison to the Australian climate so gloss topcoat tends to come out of the tin very thick to say the least.

I read somewhere else on this forum that the secret of getting a smooth finish with polyurethane varnish was to apply it liberally - does the same go for gloss provided that the surface you're coating is horizontal so that the paint can't run?

I must try that viscosity trick, it sounds interesting. I suppose you have to note the temperature as well so that any further mixes can be tested in similar conditions?

[durwood]

Piling on a thick coat doesn't mean you get a better or shinier finish.

If the paint is too thick (not enough solvent in it) it will set up before it can flow out and the brush marks or peel from spraying stays.

If you thin it correctly there is time for the paint to flow as while its wet and the solvent is evaporating it is able to move. Leave the solvent out and its so thick it dries faster as any solvent dries out from the thick paint to even drier. (hope that makes sense)

You can add as much solvent as you like it will not affect the gloss level, it will make it thin so that means it can run easier. but the thinner you make it the less paint you can put on before it runs. The only way to lose the gloss is to use the wrong thinner if you use a different, wrong or faster drying thinner the gloss may be effected. Always use the thinner recommended on the tin.

Again it doesn't matter what the paint is, the same rule applies too thick won't flow too thin, runs. The idea is to find the best ratio of solvent to the amount of paint. Most paints come needing thinning very few are ready for use, though there are some.

If you have the paint the correct thickness when you apply it it won't run even on a vertical surface unless you do something wrong. Such as applying too many coats at once, painting when its too cold. painting it on too heavy.

With a brush you can only do one coat at a time and always finish by wiping out the brush and smoothing the paint out with vertical strokes not horizontal. This way the brush marks can't run down to join with the next one and then then next etc. With spraying its possible to keep pouring it on so you have to know how to spray and when too stop. Just because the surface is flat doesn't mean it will not run you can dose it up so much it can run over the side and drip off. Poly poured on heavy may flow out smooth but it has hardener in it which locks it up solid. Try the same technique with other paints and you could have a terrible mess on your hands as the paint wrinkles, bubbles, blisters, or cracks to name a few.

10% was only a guide, your paint may be more or less than that, some need 20% it depends on the brand and quality of the material. You may also need to add more solvent as the tin empties as it will lose solvent when the tin is opened. Leave the lid off for a while and it may loose a lot of solvent and become even thicker.

You are on the right track with the temp, colder it is thicker the mixture will be. Normally the temperature you exist in doesn't get extreme as you would turn on a heater or an air conditioner but yes go outside in the snow and its going to get thick.

[woden]

Hi again durwood.

If there are no thinners stated on the tin would white spirit suffice as a thinner? And just on this point do you know which makes for better thinners, white spirit or turpentine substitute?

You mention that painting it on too heavy can cause the paint to run, is there a way of knowing when you've applied too much? Or is it just a case of experience and using your eye?

Is polyurethane varnish the only varnish with a hardener in it? Are there others or indeed paints that contain hardeners? And does this mean that when using polyu. you can slap on as much as you like and it will still all go off properly?

Again, so many questions.:o

Ps, on the white spirit vs. turps subs thing I find the former more pleasant to work with as the fumes aren't as bad.

[durwood]

Each type of paint has a particular thinner. White spirit is a fast drying solvent turps is slow. White spirit may thin a particular paint down but the end result may be a complete disaster. I would doubt if you could use white spirit successfully if turps is the correct one. The paint would fail prematurely if not at the time of application.

The maunfacturer can tell you the correct solvent but it usually is stated on the label on good brands. If thats not possible smell the paint there is some of the solvent in it to start with so you should be able to compare it to a solvent. Just don't take a deep breath some solvents will give you a good headache.

Every paint is different, normally if you brush on its one nice even coat and let it dry. then you can repeat the process but if you load it up too much it will run. With a spray gun each coat has to be thinned correctly, applied correctly and again let to set up before more is applied or it runs.

Rule is you can always put more on later but its a bit hard to remove it once you apply it.

There are lots of different paints which require hardener or catalyst besides polyurethane, you can even get a hardener for normal house hold enamel. Again different formular for each type.

Once you put hardener in a paint it has to be used within a certain time the warmer it is the quicker it goes off, 6 hours is usually the limit after that whats left goes solid, in the tin on the brush and in the spray gun clean them out before they go off or throw them away.

Unfortunately if the paint needs a particular solvent you have to wear the smell. Always paint in good ventilated areas.