What am I doing wrong?

[Spychocyco]

Just finished up my second rattle can spray job, and I'm seeing the same problem -- lots of little (and a few not-so-little) rough bumps on the flat surfaces of the guitar. I sanded both guitars to 400 grit before painting and wiped each well with a tack cloth after hanging it for painting, so I don't believe it was caused by dust or foreign objects. I'm sure it's in my technique, or lack thereof. Any thoughts?

[FrankenWashie]

I found the same when I first started, I wasn’t holding the can at the right distance. You can also get partial blockages in the nozzle which dies similar. Have you checked these things?

[Spychocyco]

I thought it might have something to do with the distance. I'm trying to stay about 10 inches or so out. The general rule of thumb that I've seen is about the height of the can. Is that incorrect?

[Simon Barden]

Normally just the way the paint sprays from the can. Getting the cans warm first does help and gets the paint flowing better within the can and up to the nozzle. One reason I bought a compressor and spray gun was to get more consistent results than with cans. But Phrozin gets great results with cans, so it can be done. But he's done it with lots of guitars and I'm sure some of his first attempts weren't much different to yours.

The propellant in the cans is in mainly liquid form to start with so mixes with the paint as it's pushed up the spray tube by the propellant that's turned to gas (it's in an equilibrium state but the warmer the can, the more propellent is in gas form and so pushes harder). But a fair bit of propellant is still liquid as it escapes the nozzle and as atmospheric air pressure is less than the pressure inside the can, you get both the propellant gas expanding in volume and the propellant liquid turning into gas; both processes that take heat from the paint spray. This makes the paint thicker and can cause local clumping around the nozzle, bits of which then break off and get carried by the spray onto the finish. If you don't clean the nozzle as you go along, then the paint clumps (all very small mind you) can build up and partially block the spray, causing obvious spluttering.

So it's one reason not to spray on too cold a day. The cooling effect of the gas expansion is added to by low air temperatures and this can cause small water droplets to form within the spray which get trapped under the paint (causing fogging and/or an uneven surface). Spraying in too warm a temperature can also help the solvent to evaporate before the spray hits the target (especially if the spray is a bit too far away) so the spray goes on dry and powdery. Also, if the propellant is running low, then the spray velocity is less, and so the chances of it going on dry increase.

The type of paint makes a difference. Clear lacquers always seem to spray well, but the more pigment the lacquer or paint contains, the more problems you are likely to have. Some pigments are worse than others depending on particle size. Metallics can be bad as the metal flake size is large compared to the size of the spray nozzle, so all it takes is the odd extra-large flake or two to hit the nozzle at the same time and you get a temporary blockage, which will mainly clear without you noticing, but can result in lumpiness.

Maybe it's all down to getting familiar with can spraying and recognising quickly when things aren't going right e.g. not continuing to spray when you're near the end of the can and the spray starts going on dry. Maybe down to always turning the can upside down and spraying to clear the nozzle after each use.

[Spychocyco]

Great info, Simon. I bought a paint for the second guitar that I thought would help hide flaws, but maybe the metallic was part of the problem. This one did come out better than my first attempt with the chameleon paint (which, in hindsight, I shouldn't have tried as my first finish), but I was disappointed in the bumps and lumps. I'll definitely remember the trick for clearing the nozzle. I haven't done that, particularly between coats.

[Dedman]

Sanding between coats is also important. If you look on solid coloured new cars you can see the "orange peel" in the paint, at one time that was unacceptable but people have been conned into believing it's normal. I usually figure on sanding off half of what I put on to get it level and smooth.

[Joe3334]

Do you have any images?

[Spychocyco]

Sanding between coats is also important. If you look on solid coloured new cars you can see the "orange peel" in the paint, at one time that was unacceptable but people have been conned into believing it's normal. I usually figure on sanding off half of what I put on to get it level and smooth.

I started off trying to sand the paint, but I quit because it was stripping back the color and leaving me with a dull gray. Any tips for doing it more effectively, or is that normal?

[Dedman]

yes, it will dull it off, the clear coats and the polishing will bring it back

[DrNomis_44]

Had the same problem when I was spray painting my Gold Strat with rattle cans, found out that it was my "technique" that was at fault coupled with impatience on my part, the weather also tends to affect the results as well, especially if there are some gusts of wind as you're spraying, definitely make sure you give each coat of paint sufficient time to thoroughly dry, something I now believe I didn't do.


I guess at the time, Darwin's tropical weather didn't really help things much either, a mate of mine who has experience with building guitars said to me that he thought that Darwin's weather wasn't very conducive to guitar building, and now, I'd be inclined to agree wholeheartedly with him.