S770 Viscosity Cup Values?

[antonylord]

I've picked up a Bunnings clone of the Star S770 pot spray gun for doing a few quick paint jobs in the shop at work (2mm nozzle).

No where in the manual does it state the recommended paint viscosity for spraying (e.g. 22 seconds, Ford #2) cup. That and the fact that there's no cup included in the box... I suspect I can pick one up (and a conversion chart) from a nearby paint shop.

I've trawled several other excellent topics and tried to Google the REAL manual for a Star 770 without luck.

I'm not quite ready for the 'paint bead dripping of a stick trick' just yet - I'm just trying to make my job setups as repeatable as possible.

As an aside - we've thinned our enamel (for benchtops) in the past with enamel thinner but suspect its drying waaay too fast on warmer days. Are we kidding ourselved with just using plain turpentine instead?

Cheers, Antony.

[Pusser]

I have tried about five paint shops with spray supplies around Canberra and Fyswick for a cup but they have all said they are a special order as most of the tradies don't use them or else they last for ever and dont sell. Seems odd as my three guns all refer to the cups but dont come with them. I have just followed the shops advice on ratio of thinners to the paint I have bought. A bit hit or miss I suppose but have been lucky so far.

[durwood]

Antony,

Do you still require an answer to your question about viscosity cups and enamel?

I tried to answer while away but the computer I was on was not connected to broardband and I kept getting cut off.

[antonylord]

Sure! Would love to know!

[durwood]

Anthony,
you have the habit of posting when I'm away but here goes.

To spray any paint you need to thin it to its best viscosity.

A Ford cup (the cup has become a standard because Henry Ford used it to measure his black paint which was flow coated onto his cars, too thick or thin and it didn't work as well as just the right viscosity) The cup is basically a funnel. The industry standard for most paints is a Ford 4 cup. this is a 150ml container with a 4 mm hole in the bottom. The thinner the paint the faster the paint flows through the cup.

It is not really the gun manufacturers job to supply the cup you need to thin the paint it depends what type of paint you want to spray with it. The best place to get a viscosity cup is to go to a supplier of auto paints to the trade if they don't have any in stock (you don't exactly wear them out) they could get one in for you but if you look on ebay you should find them advertised. Proper ones are made out of brass and cost big money but they make a simple plastic one with a replaceable brass insert for the hole for a few $$$. You can use anything to do the same job,(coke can etc) all you need is a container punch a hole in the bottom get access to a proper cup and time the paint and then run the mixture through your container. Or if you find that a mixture you make sprays fantastically then grab a container put a hole in the bottom and fill it up with the paint and time it. If you then mix future mixes up to the same time you will get the paint the same fantastic mixture you had originally.


BUT if you buy a gun you need to buy one that the gun manufacturer has made for the paint you buy. Your Star gun should have info telling you what it is made to spray.

Going back to the start, when a paint is made the chemists check out what ratios of thinner to paint they neeed to get the best result. They try different mixtures and proportions and come up with the combination that gives the best result. They then send the paint and thinner to the gun people and tell them the measurements needed that will give the best results. So the gun manufacturer gets a chart that says eg: thin with solvent to gain a viscosity of 20 sec in a Ford 4? cup.

The gun manufacturers then have a starting point, they thin the paint by putting it in the viscosity cup with thinner added until it comes out at 20 sec. They then try the mixture in their guns and alter the size of the paint nozzle and the holes for the air until they get the paint coming out best.
If its different to sizes they already sell they may decide to make a new version of their gun. If the paint works great in one of their normal pruducts they will leave things the same and advise customers and sales people of the fact.

The viscosity changes if the temperature changes (when its hot the paint thins - some guns actually have heaters attached to thin the paint as this means the paint is thin enough to paint but hasn't any thinner in it so the coating ends up thicker on the surface when its applied)

The same goes when its cold, the paint thickens so the only way to know if the paint is at its best to spray is to measure it in a viscosity cup.
Because non professional people don't know or have access to viscosity cups normally paint manufacurers put on the tins simple instructions on how to thin and apply the paint. Most of the time this works great and a lot of paints are very forgiving you can add way wrong amounts to them and they spray fine ( lacquers) but try it with enamels and you will end up in all types of trouble.

Here is an example. Lacquer is usually thinned 60% thinner to 40% paint.

New paint in the tin takes about 2 minutes or more to run through a viscosity cup, thin it down 60-40 and it takes 20 sec add the same amount of thinner again and it takes 18 sec approx add the same amount again and it takes 15 sec - Thinner takes 12 sec. You can spray any of those mixtures and get a good result as long as you see the paint is really thin. You just move the gun further away from the job or move the gun faster. You can even spray straight thinner if you want. The gun should be set up to spray lacquer. If it is it will have a large fluid nozzle (about 1.8mm) and plenty of holes for air.

If the paint was enamel then it is a different story. Thick paint about 3 minutes in the cup ( depends on how good a brand) thin it 10% thinner and its at 20sec ready to paint. At this viscosity with the right gun set up (size of holes in gun for paint and air) you will be able to apply the flatest glossy coat of paint without a run ( as long as you have gained the practice to do so. If you tip unused paint back in the tin and then get it out next day if yu measure 10% it will be too thin as you already have thinner paint in the opened tin. Do this a few times and you may just have to open the tin and use it straight to have paint the correct viscosity.

Can you see what would happen if you went over with the addition of extra thinner the paint would become like water and because it is so slow drying would run everywhere down the panel. If you used a lacquer gun you would get a bad result even if the thinning was right as the holes are too large and it would pour out and run also. If you put lacquer in a gun set up to spray enamel it sprays dry and sandy.

As for the thinner used, if you are using enamel you need thinner suitable for the weather. Usually a fast thinner on cold days and a normal thinner on hot days. Turps works (its also a lot cheaper) as it is made to thin enamel paints but its designed for even slower drying (brush) you only end up with paint that stays wet longer therefore allowing it to collect more dirt etc and slowing down the amount of work you can do.