the red scuff pads are equivalent to about a 400 grit.. thats not really enough bite for primer... it works on zinc chromate, but old paint ya want more if ya can, especially on exterior surfaces..... generally you want a 180 to 220 cut on paint for primer.. raw steel, i'll 80, zinc, scuff, fill prime... like i said, if door jambs and such are relatively peel-free, you can use it on jamb stuff, etc...
after primer, paint prep, like i said, you can do a light, flatpad, slow speed, initial cut with a DA to do your bulk initial cutting, than block it out... once the panels are perfect, it's always nice to give EVERYTHING a nice thorough scuffpadding prior to blowing off, panelwashing, masking, yada...
400 is a good grit for base... sometimes i'll 320 that DA cut and blocking for speed, then run over it with the 400 scuffpad.. i'm a little leary about giving you lots of stepsavers being a noob.. the safer approach for me to advise, is to just tell you to wetsand all your primer with 400.. then maybe a nice light, thorough scuffing.. it's safer...
if your not careful with the DA, you'll end up cutting thru the primer here and there, which is what you dont want... which brings up don'ts...
when DA-ing.. always start with the pad flat.. this ensures your using the paper up thoroughly.. the sander will wreck the paper at the edge very easily, load it up, plus it cuts different..
lets say your stripping paint with 60 or 80 to bare steel.. flatpad it till the center isn't cutting as well, then finish off the paper by tipping the DA slightly so it's riding on the outer inch or so of the pad to finish off the paper.. you'll notice it cuts WAY more aggressively when on an angle... this is fine when stripping, having raw steel as your backup.. doesn't really matter... you can't hurt it and your goal is speed... but if you just run on the edge, you'll go thru a lot of paper, thus starting flatpad..
also that aggressive "edge" sanding is dangerous when sanding primers or surfaces your concerned about how deep it cuts.. i've seen guys ruin boat bottoms doing that... you can do it on primer and such, but with a LIGHT touch...
i could go on all night here..... i know more stupid sh*t about sanding than anyone i've EVER met.. there are tons and tons of little tricks, stepsavers, techniques, don'ts, etc..
i would generally say, just plan on DA-ing your old paint... handsand everything after primer, for a new guy it's by far the safer route.. as to paper... for stripping, heavy cutting of old finishs i generally run in the 50 to 80 range, depending on whether I'm stripping, or just getting a cut for primer.... if i'm cutting for primer, it'll usually be a light, take the shine off cut with 80, than buzz over it with some 150 to 180 for the primer..
for stripping i'll usually rip just about all paint off with 50 or 60, then a thorough 80 of all the steel in prep for some zinc, etc...
i would avoid anything nastier than 50.. i do use 36 and such on occassion for things but i wont get into that...
if you insist on trying it on the primer, get some 320 or 400..
brands... eh.... there's a lot to that too.. there are lots of choices.. the various 3M's are the best.. the different gold series papers from them can't be beat.. the white and purple imperial series from them is ok, but i don't think the "economy" savings you save over the gold is worth it.. it's almost the same money and i think the gold blows it away....
there are lots of brands out now.. lately i've been buying lots of mirka and rhynalox papers.. tend to be a bit cheaper.. ask your paint supply guy... Norton is always good stuff too... i also buy DA paper on ebay on occasion... some of the off brand/knock-offs do ok, not 3M but get ya by...
