Builders' Forum Archives
Posted by David Warren on Jul 25, 2006
They are talking about sanding for two different purposes: (i) adhesion to lower layers, and (ii) getting the smoothest, sexiest finish to your brightwork.
(i)Adhesion. There is an ideal time to apply a second/subsequent coat of varnish - when the previous coat is not too "wet" and not too 'dry'. This period is called the overcoat period.
If you apply a second/subsequent coat within the overcoating period, you don't need to sand to ensure proper adhesion to the lower layer. The still-slightly-"wet" layer will chemically bond with the new layer. In contrast, if you apply the coat after the overcoating period, it will not adhere to the very dry smooth layer underneath unless you 'roughen' the surface microscopically by sanding it first.
(ii) Sexy coffee-table finishes. The best way to ensure a 'perfect' smooth finish at the end of the job is to sand lightly between each coat, to remove any lumps of dust/sand/hair/blobs before applying the next coat. Varnish goes on smoothest on a smooth surface, and will 'magnify' any lumps it encounters. This is true whether it is done within the overcoat period or not.
So, their slightly confusing directions are really saying that it is necessary to sand if you re-coat after the overcoat period, but you should sand anyway if you want to paddle a banana-shaped mirror.
Hope that helps! .
In Response to: "Overcoating" by John Pollard on Jul 25, 2006