I am not sure what the DP95 is made of, I assume you mean polyester when you say 'fiberglass resin'. It should be fine but I would suggest a dime size drop on the inside left to cure to be sure. Be sure to put it against the material of the helmet itself and not a coating material. Because the polyester is a two part catalyzed polymer it doesn't have the harsh solvents you usually have to worry about causing a bad reaction with other materials. Testing it in a small spot is the only way to be sure unless someone else can attest to whether it's a go or no-go based on their personal experience.
Regarding modifying the shape of a helmet: if you're using heat with help of, say, a heat gun, then heating the helmet and then rapidly cooling under cold running water would do the trick. I've had to do some modifications to a Rubie's plastic helmet, and this method not only works, it holds it shape well. The trick is to heat slowly and the area around the immediate area you want "retrained" so that the "memory" of its original shape does not assert itself in the modification.
Fibre glassing the inside of a Don Post helmet should be fine, I did this to my old one (now sold, too small ) I used the standard polyester (?) resin found at most auto repair shops and the glass matting that came with the set. Remember to let the strips dry fully before adding more.
I am looking to give added rigidity to my Rubies Vinyl Helmet (I'm experimenting at the moment so opted for a cheap make!). I was wondering whether brushing on layers of non-corrosive epoxy resin to areas which flex or warp would make them harder and more rigid? Also gluing a high density plastic like acrylic in the form of ribbing on the inside would prevent warping. any ideas out there as to the best way to make my helmet more stronger and rigid?
I´ve also got a vinyl helmet and I used epoxy resin. Two layers on the outside so I could use enamel for the paintjob. On the innerside I also used epoxy resin followed by some layers of fibreglass - very hard!
Please do not use polyester resin - it will heat the helmet!!!
Tazman..what was the preparation for the fibreglass? Was it simply sand inside plastic, apply epoxy resin and then add fibreglass patches (with more epoxy on top)? Did the fibreglass take to the platic or does it come aprt from it when you bend the helmet?
Score the inside of the helmet with a big kitchen knife to get some good gouges and grooves for the resin to take to. Put a thin coat of resin on the part you want to work on and stick the mat to it. Dab more resin on top until completely covered. Leave a few inches and repeat. This helps to minimise the heat caused by the curing resin. Use small strips of matting as they are easier to fit into curves etc. Once cured and cooled go back and overlap all the gaps as this will add strength to the joins. If the inside is not perfectly smooth and free from fibers dremel the hell out of it until it is.