Monday 9 July 2018
The first process in preparing for polishing is cleaning up. If the job is suitable you may have been able to polish the inside show-wood of the carcass before glue up. If so that will make cleaning glue squeeze-ut very much easier.
You might also have used either hot or cold animal glue which cleans up much more easily and does not resist colours or finishes as strongly as PVA adhesive.
If you have not been able to use these strategies, and you have not been able to mask adjacent surfaces against squeeze out, then you will have a long job papering off the excess glue.
I gave an introduction to the use of papers last month. The rule is to start with a coarse one such as 180 or 240 grit and work through the numbers up to 400 grit.
Use each paper until you can no longer see the scratches from the previous grade of paper when you wet the surface with water or white spirit. This wetting has nothing to do with raising the grain; at this stage all you are doing is using the liquid to increase the reflectivity of the surface so that you can see surface defects more easily.
Make sure that you paper with the grain, not across it.
Raising the grain
Once you have reached a stage where there are no imperfections on the surface when viewed in raking light, see panel, you can raise the grain and give the surface a last papering with a fine paper.
How fine you go is your choice but it needs to be at least 320 grit on close-grained timbers and 240 is barely enough on coarse timbers. I always work to finer papers than these but I am also accused of being a perfectionist.
You could use a random orbital sander on the coarser grits but the final papering should be by hand.
Grain-filling and sealing
If you are using a coarse timber and do not want an open-grained finish then now is the time to fill the grain. You have several choices. You can use sanding sealer, shellac and pumice or a proprietary grain filler.
I do not like the coloured proprietary grain fillers because they can tint the timber.
Shellac and pumice is the old-fashioned way but it is time-consuming. I tend to use it only for antiques and for the flat show surfaces.
That leaves sanding sealer. This is a thick-bodied polish that is applied heavily with a brush and then hand sanded back. It does seem a bit crazy that the bottle of sealer that you have paid for ends up as sanding dust on the workshop floor but that is the price that you pay for an easy finish.
One thing that needs to be remembered though is that the sealer will tend to sink into the grain so practise on a scrap of timber and decide after four weeks whether the appearance is what you wanted. It is easy enough to apply more sanding sealer.
Once the surface is sealed you can apply any filler that is needed. If you put filler onto bare timber then you would stain the surrounding surface. Of course you could mask the surrounding surfaces, but that choice will depend on the order in which you are doing things.
You see, there is a whole range of methods and sequences for applying finishing materials to timber. You have to learn how they all interact and what causes things to go wrong. That is what I am going to concentrate on next, how to assess the likely consequences of applying each particular material.
A finish consists of layers of material, one on top of each other. The timber is like blotting paper.
It absorbs clear materials and its reflectivity is changed.
It absorbs coloured materials and its colour is changed.
If the surface is covered with a material that dries flat and smooth then the timber appears shiny.
There is a whole range of adjectives used to describe the appearance of polished timber and some of them will appear contradictory, such as a flat surface appearing shiny. The only way to understand finishes is to get out some practice boards and use the materials.
By the way, a flat surface reflects light better than a wrinkled or dimpled surface, and that is why a polish that dries absolutely flat looks shiny.