Not that I'm any whiz-bang, but I just sand the bugger with 400 grit or whatever—doesn't make a lot of difference to me, fill and sand any seams, wipe with alcohol (no, I don't use my rum) and go ahead and prime it with the rest of the rocket. I figure it gives the paint something nice to grip on to. Sand as usual with the rest of the rocket.

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Retro-grouch