Estes Skylab #001973, Build, Part 12, Fin Fairings - Patience!

That vacu-form lip on the inside will have to be sanded to match the contour of the card stock crescent piece.

Notice the center mark on the cardstock crescent lines up with the (inside) center rib of the fairing.

Back to the sandpaper taped to the body tube

Sand just to match the circular cut of the card stock crescent - no more. It fits the BT-101 tube curve perfectly.

I've mentioned this is previous builds - 

The original Centuri Saturn V kit provided a Fairing Trim Guide to show the curve of the fairing sides.

While the guide says "trim sides" you'll have more control (and not remove too much) through careful sanding.

If you have sanded correctly, you should end up with that subtle curve on the sides.

I've sanded and fitted just up to the forward ends of the fairing ribs on the side edges.

Notice the fin slot line has a slight up-bulge against the sides of the fin - it's not a straight line. That tells me the fin slot isn't wide enough where the line distorts. That slot still needs more sanding to widen the slot. Go slow and check after sanding. It won't take much to improve the fit.

Look close at the fit of the fairing. 

The fairing sides should fit just inside the corrugations on the lower body wrap. The rounded tip extends over the top of the wrap.

I've seen a few finished Saturn Vs where heavy fillers have covered up a poor fit of the fairings.

The fairings should not rest on, but fit inside the ribs on the low wrap. 

If the fairings don't fit fairly easily inside the corrugation ribs, you probably have more sanding to do.