Glue question

Hello,
I'm currently building a plywood boat on a spruce frame using a set of plans from 1956. The instructions recommend filling gaps with a mixture of sawdust and glue. I've been using 3M 4200 for most of the plywood, but it stays rubbery when cured, so that's not suitable. I'm looking for suggestions on a modern material I can use with the sawdust. The joints will be glassed and painted. Any advice would be greatly appreciated!

 

If you are using a modern adhesove, use a modern filler too. If the parts are properly fitted, there should be a minimal gap. Epoxy with fumed silica filler is the usual adhesive for when there are gaps. What have you used 4200 for? Are you talking about the splices?

 

The obvious advice is to work to a standard that creates only tiny or non-existent gaps.

 

The problem with epoxy and sawdust is the bond to the substrate will suck the resins easily away from the bond margin. To use epoxy and sawdust(or cabosil); you must precoat the area with mixed epoxy no fillers first. The first pass and probably about 30 minutes and you can develop good bonds. On wood that is sun dried or aged, sometimes, I precoat it two times before adding fillers.

All work is wet on wet or green. Avoid wet on yesterday’s precoat.

Wood flour or cab is preferred. Sawdust is only used for bulk filling like the end of a canoe or some places noone sees.

 

Look up fillers compatible with your 3M 4200, at a 3M site. You could get some of your scrap plywood, cut off / shave the outer veneer to match colours and grain, then shape pieces to cover the gaps, so when you have filled the gaps with whatever recommended, and it's still wet, stick the veneer pieces in place, and 'no more gaps'. More consistent surface when sanding and painting. Disguising indiscretions.
Glues were different in the 50's and 60's, and it was normal to mix up wood powder or fine saw dust in glue for small gaps, even to make a putty for plugging sunken screw heads, but products now are not the same, so find what's compatible.