You haven't described the construction of these joints, so this is a guess.
Do not use a sander away for the remainder of the project.
Plane what's left of the members to a fair profile, even if they're
significantly skinnier than designed.
Bond to the members a strip of stock sufficiently thick to reach designed
dimension. Clean squeeze-out carefully.
Plane the members to designed dimension and profile.

ReelBros wrote:

Hello, I've built a Ken Hankinson design LaPaz22 in 24', in my haste,
and lack of a good belt sander my reverse chines/strakes have some
serious "roller coastering" any suggestions on what to use to build up
low spots, they are epoxied on so i'm doomed.
Regards,

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ReelBros