Q&A – Should I use coarse sandpaper on timber or ply to make the epoxy stick.


There  was the suggestion that very coarse sandpaper was the best to use on a timber surface to ensure good bonding when gluing or fiberglassing the surface.
This is not recommended – the following explains why.

There  was the suggestion that very coarse sandpaper was the best to use on a timber surface to ensure good bonding when gluing or fiberglassing the surface.

This is not recommended – the following explains why.

The Microstructure of Wood.

Part of any bonding to timber requires that the glue goes up the into the ends of exposed wood cells – they are like tubes. If too coarse paper or a blunt blade is used the ends of the cells get crushed rather than being clean cut – blocking the mouth of the cell so the resin can’t get in so easily with the result that the bond is weaker.

Weaker is a relative term – it may still be strong enough.  But if we know the method with the best results then we can use it!

The best bonding surface is freshly planed timber. But no-one is likely to plane the surface of a finished boat as the final surface – don’t want flat bits everywhere and it is an awful amount of labour. But it does indicate that planing tools should be kept sharp – whether a thicknesser or a handplane or tungsten circular saw blade when preparing stock.

What grade of sandpaper

The best sanded surface uses finer grades of paper. The source I have extracted the information from is (the link should be clickable – about 500kb PDF file)

Forest Products Laboratory. 1999. Wood handbook—Wood as an engineering material.

This article is well worth reading through, but indicates finer grades than 100grit and does mention 320 being used in some of their tests and demonstrations.

It is also important that the sandpaper be sharp.

Testing if a surface is suitable for adhesion

There is a neat little test that can be used to check whether the surface is OK for bonding by placing a water droplet on the surface and see if it “beads up” or spreads out on the surface.

Using water drops to show if a surface is ready for gluing

Using water drops to show if a surface is ready for gluing

An good adhesive needs three properties

  1. it has to wet the surface
  2. it has to harden
  3. it has to be able to transmit loads from one side of the join to the other.

The water droplet test takes care of the wettability of the surface. I have only heard of the test before but haven’t tried it – however I imagine that oily timbers like Teak or Australian White beech will always repel the moisture.

Strategies for Oily Timbers – Teak, Australian White Beech

At the same time there is a strong track record of these oily timbers being glued to decks in a reliable way even without fastenings – so the test is an indicator only. (as teak repels water I would not suggest using glued teak structures for any critical part of the boat without applying the appropriate fastening schedule)

Teak is commonly glued as a deck overlay and the surface is prepared for gluing by brushing some Acetone on the gluing surface and wiping off with a clean rag shortly before applying glue.


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