Each plastic is sensitive to a different group of solvents. What you have in the varnish is a solvent that is agressive to the PC resin causing the cracking.
You will need to find a varnish/resin combination that is compatible to avoid cracking, while still having enough "bite" for adhesion of the varnish.
The resin supplier will be able to provide you with a "compatability list" for the resin material, then you can select a coating/varnish that has the right mix of solvents.
(Another less likely cause is residual stress in the resin part where it has been formed. Sometimes these parts can be "annealed" to reduce the incidene of cracking by putting them into an oven near the HDT of the plastic material for a few hours then ramping the temperature down. This is an industrial porcess and should only be done with properly vented and controlled ovens.)
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Just an Engineer from the land down under.
Polycarbonates, in particular, are susceptible to solvent induced stress cracking. Coating specifically for PC's do exist so look for one. Changing to a PC/ABS blend or ABS may be an option as well.
Varnish Causes PC Resins to Crack
Re: Resin PC crack when touch Varnish
