ok.
coming from someone whos done bodywork in the passed.
Rub it down with 340. if you use a to course grade u will see it in the primer and there fore the paint. unless you use loads of primer which isnt needed if done correct
on the filling use 120 - 180 grit to rub it down, in a circular motion. ( use a rubber block, if you use fingers u put minor marks in filler...) if you need to use your hand, use your palm.
Use panel wipe ( i got 5L from local paint suppliers for 12quid) after panel wipes use a tack cloth to get any small particles off. ( panel wipe is better than water as its hydroscopic, which means drys in the air.)
after primering, flat it down with a scotch bright pad (old trick), you can also spray satin black on and flat it till its all gone and uve got a nice smooth finish on the primer. can also use wet and dry, use it wet though, as it clogs the paper dry. can also use a scotch bright then use 1200 - 1500 if your really going for a exceptional finish.
the drying times of paint is around 10 minutes inbetween coats with a decent heated place.
if you need to know anything else...feel free to add me on msn
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