How long did you let the paint dry before you applied the poly? Off hand it sounds like the paint wasn't completely dry. Remember any dry times on the can are under ideal conditions which aren't what one is lucky enough to find in the field.

This is a really old thread but I just wanted to say that I had the same thing happen to me. The first time (both were on test pieces) I had large crackles (almost as though I had applied a crackle medium). I suspected I may not have let the paint dry thoroughly enough (it was probably about an 8-hour dry time). I tried it again after a 36-hour dry time and again had crackling, but this time they were very fine and small. I sanded the area lightly, reapplied the polycrylic and it was a perfect finish.

I don't know what happened, but I have to suspect it had something to do with the paint's dry time or perhaps the conditions (I was doing this in the basement, which is cool but not cold).

This is just another reason why we don't like to mix and match products....compatability. The polycrylic is not going to add any durability that the paint itself can't provide.

So, if I painted a desk then there is no need to apply a varnish over it? The varnish would add no further protection?

PG is dead-on here...

Good "regular" paints & "Cabinet-type" paints...

DO NOT NEED additional coatings!!!

Cabinet-paints typically have Polyurethane resins in them, and don't need additional clear topcoats.
Once the paint itself is dry, CURING is still ongoing...and it CAN take 3-4 weeks to considered hardened-off.

If you absolutely have to clear-coat something...WAIT AT LEAST 2 WEEKS for paint to more completely cure.
>>> Some moisture is still releasing from new paintcoats...that's why subsequent coatings fail/crack.

Faron

Crackle is more of a drying issue. Top of coat dried faster than what was underneath. Gassing from a previously applied coating/paint will cause bubbles.