I’ld always recommend p38 ( car body filler) used it on all my boats in some form over the years, grp and wooden hulls.
And once sanded and smooth I use a fine surface filling compound.
Any decent brand will do the job, I’ve had remarkable success recently with Wilko’s own brand which is easy sand so it’s not tough as nails once cured to sand back unlike some other brands.
Please for the love of god... do not use plaster! I recently saw a boat absolutely ruined from using plaster as a filler (idiot) model was fine until he took a little knock and half the hulls surface literally fell of the model.. laughable at the time...
Anyway you can’t go wrong with p38.
Oh in a side note if you want to beef up sections on a grp hull you can also use grp bridging compound, it’s a pre mix of grp glass and resin, just add the hardener, and whack it into place and smooth out, you usually get about 20 mins work time, just take a look at my sea urchin blog and my more recent post on the aeronaut Jenny build, both have used the bridging compound and the Jenny used it as a rather brutal repair.
You can get the bridging compound in wilko for £5 along with the p38 for the same price, it also helps both are same brand so no bad reactions later on.
Used here to reinforce planked bow,https://www.thercmodelboatforum.com/t1340-aerokits-sea-urchin
Used here to repair bow and stem in a nasty gash.https://www.thercmodelboatforum.com/t1044-aero-naut-jenny
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