Terrible (intermittent) problem with Poly

[John Noell]

We are spraying a second coat of matte poly 24 hours after first coat (with light 320 sanding in between) and sometimes we get what the picture shows, In this case every single board (a lot!) that was sprayed did this. The poly is slightly thinned with turps. (I thought this was a bad idea but the spray guy showed me the can and it says mineral turps is okay for thinning.) Same can of poly as was used for the first coat (which looked perfect) yesterday. What did we do wrong?

[Phil Thien]

Looks similar to what happens when you spray rattle can paint incorrectly. That is, the cans say to recoat within X minutes, or after Y minutes. Spraying between X and Y will result in that look.

[glenn bradley]

The poly I have worked with states to re-coat in under 2 hours or wait 72 hours, sand, wipe off any sanding spoil and then apply the next coat. I don't think poly will really bind to itself in the "burn in" way that other finishes will. It needs the mechanical bond of the sanded surface. The pic really looks like something was on the surface of the previous coat tht caused the new coat to alligator. I would wait a couple days until you are sure that what is on there is cured, sand it down and go again.

[John Noell]

This poly is made right here in Fiji. The can says "Drying Time at 25°C & RH 60%: Touch dry 2 hours; Recoat 16 Hours."

I thought that meant wait at least 16 hours but maybe it means recoat within 16 hours??? Or...???

[Howard Acheson]

What you are seeing is generally the result of the thinners in the finish interacting with the not fully cured prior finish. For example, what your picture shows is exactly what happens if you apply a lacquer solvent finish over a not fully cured oil based finish.

I have never thinned an oil based finish with pure turpentine. Turpentine is the most aggressive thinner that can be used for oil based finish. It's possible that it is partially re-dissolving the first finish.

My suggest is to use petroleum based mineral spirits as your thinner. Or, let the turpentine thinned finish dry for a couple of weeks before applying another coat. Oil based finishes dry it two steps. First the thinner evaporates and the finish feels "dry". But after that, the finish begins the curing process where oxygen mixes with the varnish causing it to cure into a hard, well adhered, and protective film. It takes 3-4 weeks for an oil based finish to get to about 90% cured.

You might want to contact the manufacturer and ask them what thinner to use and/or to analyze the problem.

[Bob Wingard]

"Alligatoring" is typically caused by two incompatible finishes reacting or by surface contamination ... is there any chance that fumes or debris settled on the surface between coats ??? Anybody running a 2-cycle engine in that area ???

I usde to use Pure Turpentine, thinking it was a "better" solvent 'cause it cost more ... now, I use strictly VM&P Naptha in my finishes. It dries fast and leaves no residue ... you might try wiping down with a white rag dampened with some between coats and see if it picks up any crud.