Have been doing some work on the forearm i assembled a while back. (Forearm assembly)
I started with some sanding.
After the sanding and blowing it clean i used Polyester (fiberglass resin).
First time using it on PLA printed parts, and it really added some strength to the parts. I will actually go over the inside as well to strengthen the part. (Since the part didn't come out great from the printer).
After the Polyester (fiberglass resin) had cured i sanded it down and then used some bondo to fill in the gaps from gluing the parts together. Then i went over the forearm with spray filler, this will repeat until i'm satisfied and can move on to start priming the part.