MIMF

I recently bought an inexpensive acoustic/electric guitar which has two deep and long cracks in the poly finish running from the edges of the endpin jack down along the butt wedge but not quite reaching the edge of the guitar. This is a pretty thick poly finish (unfortunately) as we often see on inexpensive import guitars, and the depth of the cracks fully reach down to the wood surface.

I think the instrument sustained a hard knock to the endpin jack that created the cracks, but I don't know for sure. I took some internal pics and it doesn't look like the end block sustained any damage, so I'm only worried about cleaning out and filling this narrow strip between the cracks. I can't take a picture unfortunately right now, but imagine two cracks that run parallel from the sides of the endpin jack (and just slightly wider than the width of the jack) down about an inch or an inch and a quarter, but not to the edge or even quite to the back binding. The area above the endpin jack is fine...no cracking there. The cracks are puckered up from the surface level and could easily be picked at with your fingernail (or better said, crack lines are not at all flush with the finish).

I was thinking of either cutting out the whole area with a sharp exacto knife and filling with epoxy, or just cutting along each edge of each crack and filling those thinner strips instead. If I cut the whole area out, the plastic butt wedge south of the pin will effectively be fully exposed if you can picture that.

Alternatively, I could just try to wick in CA and close the cracks as best as I can and see how much I could sand/buff out. I have never messed around with a thick poly sort of finish, so not sure the best approach here. I assume that filling with nitro will not burn in any of the poly to blend?!

As you can guess, I'm pretty inexperienced on these sorts of fixes . But I want to try it myself...and even if I mess it up, it is a cheapo guitar to begin with...Any thoughts?

I have no experience with poly, but I think the CA is probably your easiest solution. Another option is to use Polyester. Polyester is the same resin used for fiberglass. Buy a small kit, clean the crack as best as possible and fill with the resin (possibly diluted with acetone), scrape sand level and buff.

On a cheap guitar I would just wick in some thin CA. When that has filled the cracks (several applications) I might scrape the uneven edges with a sharp razor blade and shine it all up to the best of my ability. Keep in mind that CA is a lot softer than polyester and a little softer than polyurethane. If you buff vigorously the CA will end up lower than the surrounding areas. Also CA will shrink for at least a week as it gasses off and that will lower it as well.