Using Sanding Sealer

Collated from newsgroup postings.

I have often seen references to people using sanding sealer on their work. What does it do and in what circumstances is it used?

It seals (and therefore protects) the timber. It fills the pores and hardens the fine ends of the grain so that when you sand it, the grain sands away smoothly. It provides an excellent base for finishes such as polyurethane, varnish, wax etc. I use Feast Watson sealer but there are plenty of other brands. When turning, I rub it on the work with a small piece of rag then turn on the lathe. I rub it into the turning work until perfectly smooth. Then I let it dry, sand while still on the lathe and then add whatever finish I want.

You will find that if you do not use sanding sealer that the first coat of your finish will raise the grain and then you have to sand off something that may be a lot harder than sanding sealer. Certainly on turned articles I find sanding sealer to be one of the keys to a really good finish. Its not that expensive, give it a go and see what you think.

I use Mirotone sanding sealer. I rub it on with a rag with the lathe stopped then cut it back with either 600 or 1200 grit paper. It makes an excellent base for the final finish.

Really there is no need for sanding sealer at all. All polishes are there own sealers. The first coat of any product 'seals' the wood. The main advantage in using a sanding sealer is to make sanding easier. I use mirotones lacquer and also use the corresponding sanding sealer - but the only real difference between the two products chemically is that the sanding sealer contains talc to make sanding easier. I would think that if the sealer you are using is clogging up the paper rather quickly, then its not doing its job properly, it may be on old batch would explain it not fully curing - which is what sounds like is happening.

I don't think that all polishes seal well at all. I've found that many waxes, oils, polishes etc. look great for the first 6-12 months but then go dull. Sanding sealer actually seals the timber, like a varnish or lacquer, and also forms a really good base for wax and polishes. I've never had any trouble with it clogging up sandpaper, probably because I always use it sparingly and really rub it into the wood. When I use it on turnings, I rub it in like a friction polish.

Waxes etc on raw timber will dull off. That's OK they're supposed to dull off, when this happens it means it is time to apply a little more wax to the surface. This will help nourish the timber and keep the work in tip top condition.

I suggest that if you intend to use a sealer that you would probably be best with a very weak shellac solution 8-10 parts metho to a premixed shellac. (preferably dewaxed white shellac) This will raise the nap of the grain and make it stand up and become reasonably brittle. This can easily be sanded using a very fine grit (600-800 or even finer) then a wax can be applied. This will keep the shine a lot longer.

Too many woodturners use sanding sealer the wrong way, as a finish. IT IS NOT A FINISH just a means to an end. It fills the grain and in many instances hides a multitude of sins. Most top class woodturners wouldn't touch the stuff except for use on their lower quality production work which will end up being sprayed.

If you are using a friction polish like Shellawax Cream the use of a sanding sealer will hinder the final finish as it needs to be burned into the timber during application thus fusing itself into the wood rather than becoming a surface coating which is much easier to damage. Sealer will also dramatically hinder the application of most oils.

You must be patient with your finishing. Most experienced wood turners will tell you they spend more time finishing i.e. sanding than they do actually turning. I have a tin of sanding sealer which I have used only once. There is no substitute for very sharp tools for that final cut followed by careful sanding right down to 800 or 1000 grit followed by at least 2 applications of EEE, and then if you want it really shiny Shellawax Cream or liquid.

Sanding sealer is NOT a finish, it just provides a really good base for various finishes. Not everyone actually enjoys re-polishing all their stuff every few months, or even every year! Sealer also helps to keep timber stable, I believe. I'm not sure how long you should really wait to add finish after putting sealer on like friction polish. It seems to dry straight away. I usually leave it an hour or two but that's probably not really necessary the way I use it. By the way, I also agree that sealer is not a substitute for fine sanding. Sharp tools, rub that bevel, sand to 300-400 really well, then use sealer, then use wax, Danish oil, poly, boot polish, or whatever.

Sealer is a means to an end. From my experience sealers are a lacquer based product anyway. Thats why there is no need to continually 'Feed' the finish as you would with waxes and oils. But in reality even using a sealer and then polishing with a wax or oil, you still should reapply the wax or oil occasionally as because it hasn't been 'Fed' into the timber it will eventually wear off the piece leaving nothing but the sealer behind left to protect the timber.

Everybody has there own preferences in everything we do. There is no one way to do everything - especially when it comes to polishing.

Personally the only time I use a sealer is when I am finishing with lacquer. Whenever I use waxes etc I either use them as sealers or a reduced mix of Shellac. Shellac is actually universally renowned as the best sealer - but how many people use it as such?

What exactly is it? Is there an easy way of mixing it oneself?

Sanding sealer is just that, a material that seals wood before sanding. The idea is to harden and raise the fibres so that they are easily cut off and to sand with less dust and faster cut. I just went out and bought a can of sanding sealer and tried it. I prefer it on guitar bodies for orbital sanding as opposed to lathe turned objects but that is just a personal feeling. You might want to try using your favourite finish cut about fifty percent with turpentine as a sanding sealer to see if you like the idea.

Peter Child in his classic work on turning says that he mixes talc with cellulose thinners and this produces a sanding sealer. Equal volumes of each is what he recommends.

Try a very weak shellac mixture (about 1 part shellac flakes to four parts or more alcohol), this mix dries very quickly, seals the timber and prevents further surface coatings waxes etc. from soaking in. It also causes any wood fibres bent down during sanding to stand up and when the shellac dries are easily "de-nibed", or sanded off. Shellac is an excellent base coat for most known finishes and has the added advantage of being cheap as well.

There are commercial products sold as sanding sealers, intended for particular final "types" of finish coats. For example, there is a "lacquer sanding sealer" for a lacquer finish, "water-soluable poly sealers" for water-borne finishes, etc. In general, a sanding sealer is just a thinned down coat of a finish intended to soak into the wood, and harden the fibres prior to your smoother fine sanding efforts. Some of them contain stearates to make the sanding dust "slippery" and less likely to plug up the sandpaper so quickly (the same thing used in the manufacturing of "non-loading" open coat sandpaper).

However, you can make a sanding sealer from the same thing you use for a top coat by just adding a compatible solvent to make a thinner version. If you are using a finish that would normally have an adhesion problem with your wood choice (such as lacquer on cocobolo), a thin coat or two of shellac (dewaxed) works both as a sanding sealer and as a barrier coat to which the lacquer can adhere, while isolating it from the wood, itself. But, in general, a sanding sealer is just a thin coat of finish that will penetrate deeper than the final finish, while hardening the wood fibre to allow your final sanding effort before moving on to your top coat(s). Any grain raising will have taken place by the time you sand down this sealer coat. The key is do your final sanding lightly, with fine grit paper, trying not to penetrate the sealer back into bare wood (that's why you use something thin - deeper penetration). Shellac is usually a safe bet for a sealer if you use the blonde, dewaxed shellac. It will, however, add a bit more of a colour change to the wood than a water-clear finish would, even if it is the super-blonde shellac, more so to very light wood than darker woods.

Surely a sanding sealer has an additional ingredient that prevents the paper from filling up and so enables the paper to do its work for a longer period and thus make sanding easier. Does any one know what the ingredients are? Is talc still used or is it a form of wax? Usually if you allow any sanding sealer to stand, this 'filler' ingredient is seen to settle to the bottom of the bottle. I doubt if it is just a thinned or cut version of the standard sealer, no matter if it is cellulose, lacquer, shellac or whatever.

This "missing" ingredient that reduces the plugged up sandpaper is stearate, a derivative from animal fat, or tallow, often used in making soap. This is also the thing used for non-loading open coat sandpaper. A sanding sealer needn't contain this to be considered a sanding sealer. The main idea is a thinned down coat that penetrates the wood, does any grain raising, and hardens the wood fibre prior to final sanding, but before applying the main top coats. If needed, it can also provide a barrier between the wood and a finish that might react adversely to the resins or oils in the wood (dewaxed shellac usually works good, in this situation).

The operative word here is "sealer" which only does what says, seals the wood to prevent further penetration of any finishing materials. Anything that is added, whether it be a stearate or raw talc, acts as a lubricant to prevent the sandpaper from becoming clogged when the sealer is sanded.

A "filler" leaves something behind that actually fills the pores and open grain of the wood.

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