I start with 80, and always use a block. Obviously a standard sanding block is too big, so I custom-cut one for most applications out of 3/8" balsa. If you use the self-adhesive file paper you can just cut and stick it on. Get it all straight with 80, then finish with 180. One final thin coat of filler sanded with 180 will get 90% of the pinholes. Save inside and outside radiuses for last. Leave the edges sharp until the very end so everything is true, then create consistent radiuses. Straight inside radiuses can be made perfect by using a chunk of brass tubing as a form.
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Hope this helps.