Plastic screw cap restoration

[Rich C]

Having had no luck finding a replacement screw cap for my vintage Century, I decided to restore my original.  Luckily it was cracked only at the top horizontally and not vertically through the threads, but still screwing it down would put pressure on the repaired area so it had to be a secure, strong repair.  I first tried gel super glue which worked at first, but the first time the lamp heated up it let loose.  I decided to then use a super strong epoxy I had from repairing metal and plastic model cars.  I've used it successfully to repair various kinds of automotive parts in stressful locations and wide variations of hot and cold.

Anyway, here is what a started with, a cap with a crack about 180 degrees around the top of the cap.  I had already cleaned out all the leftover super glue residue.  It does not stick well to the bakealite plastic the cap is made from

I wanted the epoxy to get a good grip on the plastic so I sanded the surrounding area to clean and slightly rough up the area to be glued.  The cap had sat cracked but screwed down for a couple of decades so the crack was permanently spread open at this point so I was able to also clean up inside the crack itself which was critical to getting a good bond.  I scuffed the rest of the cap knowing it would help with future paint adhesion.

Here I have applied the epoxy.  I made sure to spatula the adhesive into the crack and the surrounding area.  I tried to get it into critical areas but didn't worry about making it pretty since I knew I'd be sanding it smooth again anyway.  A little too much applied at this point would be good and help build a solid repair.

I first tried to gently clamp it in my vice out in my garage to help close the crack and allow the adhesive to cure the crack closed.  Once clamped I smoothed out the epoxy that was squeezed out of the crack.  I let it sit over night in the vice but it had not cured at that point probably due to it being so cold out in the garage over night.

It then dawned on me to just use a small hand clamp and place it near a heat register so it would fully cure sometime in my lifetime.  I wanted to clamp it really hard to make sure the crack was held closed, but knowing the plastic is somewhat brittle I clamped it just tight enough to close the crack an no more.

Here it is all cured and sanded smooth.  The epoxy cures to a hardness very similar to the plastic of the cap so sanding was quick and easy to it get smooth and even.

Instead of shooting it with a coat of primer prior to painting it, I first sprayed it with a light coat of adhesion promoter which is used to help paint adhere to plastics.  The promoter has the side benefit of being self leveling so it helped fill any tiny remaining imperfections.  It cured pretty fast so I then gave it two light coats of gold paint and it turned out great.  The gold paint I had doesn't exactly match the original Lava used but it really close.  It is a touch more shiny than the original paint but I am thoroughly pleased with the results.  The ultimate test will come when I reinstall it on the lamp and run it through a couple of on/off cycles.  I don't plan to crank the lid down like the factory did, just to snug it enough to make an air tight seal.  I think that's why so many of these crack, the factory really tightened them down way too hard.

I'm going to let it cure for another day or two before I reinstall it just to insure the epoxy and paint have ample time to fully cure.

[Rich C]

Well, one all evening and overnight cycle and the cap survived so far.  I haven't sanded and painted it yet, couple more cycles before I feel confident it will hold up.  This epoxy is VERY hard so I think it is going to do the trick.  Will be a PITA to sand down though!