Ok here it goes for plastic repairs you need to use a flexible smc 2 part epoxy, either Duramix ,fuzor or 3m. With these products you dont need to use any backer as long as youre not trying to create a missing piece of plastic otherwise you need a smc fiber backing material. As for the filler/ glaze coat i reccomes usc icing glazecoat and marson platnium plus filler. For fiberglass repairs i use Duraglass and for metal i like to use USC all metal filler. Dont even waste time with spot glazing putty as it is old technology made for laquer primers. I use all Glasurit and r-m products from BASF. House of kolor for candys and customs and some stuff from alsa corp for other additives. I would strongly reccomend using a 2 part primer nothing out of a rattle can as it is all laquer based. You need to use a good eurethane and if its bare metal use an epoxy first