Spray painting woes. Cracking to be precise.

[flanajb]

I sprayed a piece of mdf with Halfords gray primer and then applied a Rustoleum gloss back coat to a piece I am making. For some reason, the gloss back coat had fish eye marks. Strange, as the primer was fine and I had no silicone contamination. I sanded the gloss back coat partly back and then reapplied the gray primer.

It started cracking within seconds. This is what happened (ignore the circle, that's from my finger).

Any ideas what has happened here and what is the best way forward as ideally I don't want to have to sand right back to MDF and start again.

Gutted, as I spent ages sanding and fairing the mdf part in a bid to get a cracking paint job on it.

 

[mailee]

It has got to be a difference in the primer to the top coat. The primer would be a nitrocellulose base and I don't know the Rustoleum paint but would think it is not of the same base. (probably oil based) Cracking and crazing occurs when the substrate is not hard enough which can happen with differing types of paint. I am afraid it looks like it will be a case of stripping it all off again and using a primer and top coat that are compatible. You could of course use an isolator coat between the primer and top coat but these can be hit and miss. Use the cellulose primer and a cellulose top coat. Sorry this is not what you wanted to hear. HTH.

 

[flanajb]

It has got to be a difference in the primer to the top coat. The primer would be a nitrocellulose base and I don't know the Rustoleum paint but would think it is not of the same base. (probably oil based) Cracking and crazing occurs when the substrate is not hard enough which can happen with differing types of paint. I am afraid it looks like it will be a case of stripping it all off again and using a primer and top coat that are compatible. You could of course use an isolator coat between the primer and top coat but these can be hit and miss. Use the cellulose primer and a cellulose top coat. Sorry this is not what you wanted to hear. HTH.

Not what I wanted to hear, but thanks for explaining the issue. I assumed (wrongly) that all aerosol primer and top coats were compatible with one another.

I least I won't make that mistake again!

 

[mailee]

Yes sorry to be the bearer of bad news mate. It would have been better if you had used the car paint aerosol for the top coat too, they would have been compatible.

 

[Mike.S]

in a bid to get a cracking paint job on it.

Lol.

FWIW, and as an inexperienced sprayer, I had similar issues using a HVLP to spray w/b top coat over w/b primer - both by Morrells. This was on a MDF unit, with edges sealed with 2 part filler. Primer went on fine - good finish. Sprayed top coat (primer definitely dried) and got something that looked like fish eyes. Did some research and apparently fish eye appearance can be caused by contaminants e.g. silicone but also by too high pressure or holding gun too close - resulting in air bubbles forming in teh paint which appear as small 'fish eyes'.

I also got the crackling/splits evident in your photo on the edges only. Wondered if the filler solvent might have had an impact but surely this would have shown in the primer. Then I recalled that I'd changed the spray pattern from a fan to a circle (as it was just 50mm edges) without adjusting the flow and concluded that I'd put too thick a coat on. The surface dries more quickly, giving a dry film which cracks when the underlying, wetter, paint subsequently dries out. This all based on researching on the internet and from my own observations of what I did.

This may have paralells with what you've done, albeit using spray cans rather than a HVLP set-up. I'd suggest sanding back when fully dry and trying multiple thin coats, each allowed to dry thoroughly (especially in these coooler times).

 

[mailee]

Fish eyes can be pretty common on water based. It is very finicky with cleanliness.When I was in the motor trade spraying we used water based base coats and had to add silicone inhibitor to the pot as we had a lot of problems with fish eyes, this was in a purpose built booth for water based too. This is one of the reasons I use solvent instead....it's easy. Also as you found it is exacerbated by laying too thick a coat on and you are correct the top dries faster than the underlying paint which can then crack up. :wink: