I've come up with what seems to be a good easy sand bulk filler 1 to 1 plaster board joint cement and board finish mixed with a good blob of pva and a hand full of cement to aid the setting time. it mixes up well and has the same texture as normal body filler sands really well when bone dry give it a good coat of pva it sets like rock. be warned only pva after you finished sanding other wise it dam hard work. After some thought I have changed the rear end of the car I was just not happy so see if you can tell the difference.
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I pulled the rear end off to flip it over to glass around the window
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