Six Questions about Varnish Woodwork

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
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Six Questions about Varnish Woodwork

Postby dleopoldi914 » Sun Oct 07, 2018 3:56 pm

I badly scratched the mahogany wood of the anchor locker [of a Boston Whaler MONTAUK boat under discussion]. [The former] owner said in the off-season he would sand and apply coat of varnish.

Q1; Will sanding and varnishing fix [the badly scratched wood]?

Q2: What grit sandpaper should be used?

Q3: Would you clean?

Q4: Then sand?

Q5: Then rub with tack cloth to get rid of dust?

Q6: Then apply varnish?

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Re: Six Questions about Varnish Woodwork

Postby jimh » Sun Oct 07, 2018 4:21 pm

If woodwork has a thick build-up of multiple layers of varnish, and if the scratch in the woodwork is not so deep that it penetrates through all the varnish and goes into the wood itself, then the application of one new top-coat of varnish may be sufficient to fill the scratch and restore a nice finish to the woodwork. The greater the depth of the scratch, the more likely that additional new top coats may be required. Rather than try to predict how many top coats will be necessary to restore the finish on the particular scratched woodwork under discussion which is unseen and only described as "scratched badly," I suggest you try one additional top coat. If that is not sufficient, try a second additional top coat.

That the wood is mahogany does not affect this process.

As you seem to be already aware, before applying an additional layer of varnish to woodwork that has already been coated with many layers of varnish, the typical method of application of the new layer first employs some sanding of the present top layer so that the new varnish layer will better adhere to the old varnish. I cannot offer any first-hand experience or suggestion about the proper grit of the sandpaper, and I would suggest you consult some woodworking varnish experts for their advice. As a general observation, I would anticipate that the grit of the sandpaper should not be so coarse that the scratches created by the sandpaper are so deep that they cannot be filled by a single new top coat layer of varnish. As a guess, I'd suggest a grit of perhaps 400. The opinions of others, particularly experts, may differ.

It goes without saying (or asking) that with any application of any sort of finish, and particularly in the case of a clear coat final finish, the top coat must be applied to a surface that is completely clean, free of dirt, soil, and oils, and free of dust, otherwise the new layer of finish will not properly adhere to the surface of the old layer and any dust is encapsulated and preserved forever in the new top coat layer and will detract significantly from the appearance of the work and particularly if the final top coat layer is a clear finish.

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Re: Six Questions about Varnish Woodwork

Postby OldKenT » Mon Oct 08, 2018 6:37 pm

If you are able to contact the prior owner, find out specifically what varnish was used for the current finish, and use the same. Varnishes vary in color, gloss and solid content, so achieving a uniform appearance in the finish will be more likely if you use the same varnish as formed the current finish. Even varnishes marked as semi-gloss or satin vary.

As Jim states, you need to use a fine sandpaper. I would suggest that you feather the edges around the gouge with progressively finer grit, starting at 150 or 220 and going to 320 or 400. Nothing finer than 320 or 400 is necessary. If the gouge is into the wood, it might be best to refinish the entire face of the board after sanding it uniformly.