Spraycan/Paintcan compatability...

[Batpig]

Good Evening...

Over the last couple of days I've been painting up this little metal table frame that I made as part of my Dust Collection setup, and this afternoon I ran into a bit of a hickup...

The first coat I put on the bare metal was a Septone product called Rust Prime, which is sort of water-based, and leaves a black coating on untreated steel very similar to that which you find on normal stock sections when you buy them. No problems. Prefers to be coated over with an enamel, rather than acrylic, undercoat.

The second coat was an enamel undercoat applied out of a can with a brush. Again no problems.

The third coat was a coloured Dulux high-gloss enamel topcoat, also out of a can using a brush. Once again, no problems. The can was several years old but appeared to be in excellent condition. It was applied late yesterday afternoon. As far as temperature is concerned, it's starting to get just a touch chilly here in Brisbania at the moment.

That brings us to this afternoon. Unhappy with the colour of the said previous brushed coat, I tried going over it with a can of el-cheapo "Export" brand enamel spray paint from Supercheap. Problems! The effect produced was that of "Wrinkling", which is usually caused by incompatability of paint types, as is probably further evidenced by the rapidity with which it occurred.

The obvious question begs - why would enamel spray paint be incompatible with enamel paint-can paint? Both can be cleaned up with Mineral Turps...

For what it's worth, the spray paint is good. I had success with the very same can over a coat of sprayed primer just a couple of days ago on a different piece of metal.

Any thoughts anyone?
Thanks,
Batpig.

[durwood]

There are two posibilities.

!) Enamel will wrinkle if you apply too many coats. the normal number is two if you brush each one should be allowed to dry before the other is applied.

If you spray you arn't actually touching the wet sticky coat so the temptation is to keep pouring it on.

Put on more than the two coats at once ( usually when you spray enamel you apply one coat and then wait till its tacky and apply the final coat) and you risk wrinkling.

What happens is the enamel dries by being cured with the oxygen in the air. (think skin in the can) The paint being so wet still has solvent in it trapped underneath the skinning top layer. It then attacks the skin trying to soften it, it won't soften it swells up and you end up with wrinkling.

If you need to apply more than 2 coats of enamel you must stop after the second coat. let it set hard ( more than 24 hours and then you can apply more (2 coats again) usually this is only necessary if the colour is lousy at covering over whats underneath. Bright yellows, dark blues and maroons are good examples.

Normally a ground coat of a good covering colour is applied first in these cases.

There is no need normally to apply more than 2 coats of enamel, if you do you run the risk of wrinkling and you don't get any actual advantage as the paint is designed to do its job with only two coats.

2) The el cheapo can paint also is probably a fast drying enamel which means the solvent would be something stronger than turps. If this is the case the semi dry enamel coats you had previously applied would be an easy target for the solvent to wrinkle it as they would not be cured enough.

I used an el cheapo pressure pack clear applied 4 coats over a period of a week then rubbed it with 2000 paper to find it rubbed through to the undeneath layer after about 3 rubs. when I checked it out by spraying some on a piece of glass I found the clear was neally all solvent and the clear was virtually non existant ( didn't help shaking it up)

Lacquer thinner thins enamel paint no problem so I think to get the cans to be quick drying the solvents are more lacquer than turps to get the fast dry and they can attack enamel no matter how long it is allowed to dry. They are a good way of stripping off enamel as they destroy the enamel by wrinkling it up but dry quickly so you don't end up will a soggy load of goop just paint which has lost its consistance and can be easily wiped off.

[Batpig]

Thanks Durwood,

Great reply as usual. I think it's probably more a case of your second explanation, in so far as something stronger than Turps is being used in the Spray Paint to enable faster flash-off, because you can recoat after only 30sec or so with it, and cleanup can also be done with General Purpose Thinners in addition to Turps. I'll just have to wait another day or so until the single brushed coat has dried a little better, and then have another go with the can.

The reason I hit it with the can after only a day or so is that I was wanting the brushed coat to still be soft enough for the sprayed coat's solvent to "etch" into it and provide some grab between the two, because the brushed coat was very shiny (ie. not much mechanical adhesion on offer for the next coat). I didn't want to sand to get mechanical adhesion, because my instincts told me the first coat was still too soft. I thought that the timing would be just right for an expedient "chemical" solution to the adhesion problem...

When you think about it, the amount of elapsed time between coats is a bit of a juggling act: as far as I know, none of the manufacturers will say to you that you can paint another coat of enamel straight over an existing coat that is any more than so many months old, unless you undercoat again beforehand. I interpret this as meaning that you've obviously got to hit the first coat early enough for the solvent in the second coat to be able to bite into the first coat. On the other hand, recoat too early and you run the risk of either wrinkling from too much softening of the existing coat, or crazing from excessively dissimilar rates of drying of the two coats. At least that's how I see it, but I'd be interested in hearing from others with more knowledge of enamel paint than me regarding the maximum possible time between enamel topcoats...

Thanks again Durwood,
Batpig.

[durwood]

Hi Batpig,
You are a bit off base with your reasoning.

You can not get adhesion from a new coat onto an old coat of enamel by the solvent melting into the surface. The enamel sets and goes hard. The solvent if it is too strong attacks it and destroys the coat of paint from a hard coating into jelly. - "wrinkling". Its original solvent (turps) won't soften it.

You can apply a thick coat of brushing enamel one coat at a time just let each coat cure properly. Depending on the quality/brand after 24 hours you can apply the second coat. In normal circumstances the paint should be dry enough but it may still be soft though way past collecting dust. You can then apply the second coat. The job is done (normally) you don't rub it back or wash it with a cleaner just paint. Enamel has excellent adhesive qualities. Normally if you want to repaint something you clean the surface to rid it of dirt/wax/grease etc and paint on the enamel.

You can rub it back to remove the gloss but rubbing back would be more useful if you were rubbing out faults in the old paint like chips/scratches. You can rub if you want to and it will give a better job but if the old paint is smooth and clean the enamel will stick perfectly. Usually painters just sugar soap to clean the enamel ( this also removes some gloss) and then they brush on another coat.

If you have ever seen adds for cheap car resprays, it is for enamel paint. Its cheap because there is no need for any great amount of preparation.

Mask it up, wipe over the surface and spray. The enamel will stick, the glossier the old paint the better it helps the new paints finish. Doesn't matter what type of paint is on the car the enamel will go over it without any problems( the solvent is not strong enough to attack any of them) Spraying enamel is used because of this. The enamel paint will be wet for 20 minutes so any overspray will land on things around the spray area and stick. Often people spray things outside in the wind and property hundreds of yards away gets covered in small lumps.

A few years ago the railways painted a bridge near Wollongong over 30 cars in the streets around were covered in green overspray. when it dried it had to be scraped of the windscreens with a razor blade. Some of the cars were compounded but most had to be resprayed as the only way to remove the paint was so sand it off. -it sticks!!

Normally the paint process requires you to prepare the surface so the finished coats produce the best outcome. It depends on what you are painting and what standard you want. So a manufacturer will say

Clean the old paint to remove contaminates..

Sand to remove imperfections. - as you are painting again the old paint will probably be lousy so it may have chips and deep scratches in it. When you sand you will probably rub right back to the original surface , so undercoat will be needed to level the surface and seal off the wood/steel/whatever.

Sand the undercoat to get a good surface for the colour coat.

Apply a new coat of paint.

You only need the undercoat if its needed to do what its designed for If the old paint surface is good with enamel you can skip it.

With enamel its expected that every coat applies whether its primer, undercoat or colour must be allowed to dry before the next coat.

Not apply the following coat so it can bite into the first coat.

With brushing or rolling paint you have to put one coat on at a time and let it dry before you apply the next coat. If you try to put on more you mess up the still wet paint and if you overload it too much it runs initially, or wrinkles later when it starts to set.

If you spray you are able to keep pouring on more coats. Again go too many it runs though its possible to put on many coats ( I've put on 20 over 3 hours- but it naturally wrinkled later on) It didn't run as each coat will dry out enough to allow another to go over it without joining it and running. Spraying enamel dry too fast to brush easily by the way.

A good rule with all paints is "You can't apply it too slow but you can apply it too quick."


You may be confusing enamel with lacquer where the solvent melts the previous coat to gain adhesion. BUT the same rule applies the longer each coat is left to dry the quicker the end number coats will be dry. Put on one coat every half an hour and 6 coats will be far harder 24 hours later than if you put 6 coats on with a minute between them. The solvent in the first coat will be unable to dry out because its being covered by a new coat and having solvent softening it again. Let it dry out for a long time and the next coat will soften it a bit and gain adhesion but in the end all the coats will be far drier as they have had time to loose their solvent.

Next day one is rock hard and the other is still soft as it tries to dry out.

Even if your paint is left to dry rock hard for weeks the thinner in the spray can may attack it. One of the problems with enamel most other paints ( their solvent) will attack if its on a surface. Only exception is water paints turps is the bottom of the strength list all other paints use stronger solvents which usually attack the enamel..

[Batpig]

Superb reply Durwood. A genuine "Keeper"...

I certainly was off track. I might have to load up the Spray Gun for the next coat by the sounds of it, and do both pieces in the same colour (ie. the colour that was applied to the table frame by brush). I would therefore be going over the first Spray-Canned piece with Turps-thinned paint, which should be okay by the sound of it. I might do the thinning with Premium Thinners, just to enhance the Flash, but I'll keep my eye on it's side-effects by doing a test patch on the table frame first, and then moving to the other Spray-Canned piece, and when that is finished, having a close look at the test patch on the frame to see if there is any wrinkling evident. Remember, the paint will still be mostly Turps-thinned as it was in the parent can. If failure results, I'll will learn from it.

Thanks again for an obviously massive effort so late at night. I think I might send you a "Greenie" for the sheer selflessness of it...

Best Wishes,
Batpig.

[durwood]

is the premium thinner "enamel" thinner?
If it is no problem, but if its a premuim "lacquer" thinner you will be courting trouble.
Don't forget the slower the drying time the better the gloss

[Batpig]

Dear Durwood,

They are lacquer thinners! Luckily, though, I got scared beforehand and just used the brush out of the can again for the second coat (I just don't have the time to cope with Stuff-Ups at the moment......Which is worse: the Stuff-Up, or not even having enough time to risk making it?...)

You were right in your first post about Dark Blue needing two coats. Looks pretty good now after the second one.

Best Wishes again,
Batpig.

[durwood]

you now have a job with 2 coats -finished.

if it messed up you would be back to square one. No point really.

pick a paint system follow it through -job done

If you need to do it quicker than the paint drying time then change to a different paint.

you can paint some things in 10 minutes - dry hard and use from bare metal- powder coat

or you can take a week to get there - varnishes.

even some lacquers which take a few seconds to be dust free ( or dry before they get to the job if the gun is held too far from the surface) can take a month before they are completely dry.

If you have to do a dark blue again and it covers poorly, put a coat of black on first. It will then cover in one coat.