Picked up a 1930 Agway diecast truck recently, but didn't realize it was a bank. I'm not fond of diecast banks for my layout and had to figure out a way to convert it to a smooth roofline. Obviously, the problem is that there's nothing for any filler to adhere to in the slot and it would just fall to the bank floor.
I used an old drywall patch technique and cut a small piece of .060 styrene a little thinner than the slot and about 1/4" longer at each end. I then reamed a hole in the middle with a hobby knife and inserted a tiny screw for a small pair of vise grips to grab onto.
After a couple of trial insertions of the styrene into the slot to make sure everything fit, I put a small dab of fast setting epoxy at each end of the styrene piece and, using the vise grips to securely hold it and not drop onto the truck floor, I inserted the styrene into the slot and pulled it tight up against the underside of the roof of the truck. I then carefully turned the truck over so that the weight of the vise grips held the styrene piece in place against the underside of the roof and waited 5 minutes for the epoxy at each end of the strip to set. Once set, I released the vise grips and removed the screw and let the truck sit overnight.
I then started applying filler into the hole (I used Testor's model putty) and kept filling and sanding (up to 1000 grit) to get the slot perfectly smooth and level with the surrounding roof. When I was satisfied that it was level and smooth, I applied a coat of grey primer and re-sanded any high/low areas and primed again. I then applied two (2) coats of Rustoleum Gloss White paint.
P.S. - the white "ring" in some of the pic's is just a reflection from the overhead LED light.
