Before sanding try to pull down with screws, set it so the base is level first unlike what you have in the photo.

If you just sand away, without pulling down with screws you may end up with the opposite scenario of the base sitting too low.

I sand both ends a little, then as Elmo said fit it and tighten screws, if still proud repeat. If you keep sanding and hand fitting there's a chance you'll sand too much and the fin will be recessed when the screws are tightened.

Rub the fin base with chalk and then install the fin. The area that make contact will transfer.chalk from the fin base to the box. Sand those areas until you achieve the level of fit you desire.

Right then it’s easy. Sand front and rear tapers evenly with a block - no freehanding it.


but like others said do not test fit by hand it will need screws to pull it in
else you will be low when tightening it up

^^^ chalk was already suggested by jd. Crayon works too

but it won’t be high spots, it’s just oversized.

SurferKris said..
Hard-soap works as a high spot indicator too, but that is more for the two sides and it is not usually needed for the two tapered ends.
Sometimes the black plastic of the fin-head is good enough as an indicator on its own. It gets matte/dull after sanding and then turns more shiny at high spots after a trial fitting in the board.

Agree. Just look for the marks on the fin base after fitting and that’s the spot the focus on.

First make sure the fin head is not too deep. Zooming in on the pic ,I notice the back end of the base is round with some file marks. It should be square with rounded corners. Has someone previously built up the head and badly fitted it into a oversized box ? I would start with filing the rounded point of the head to match the square profile of the box. Slowly , slowly, little by little using the screws.