Finishing for a Coffee Table

[Neil Hodge]

Hi, all. I am on the horns of a dilemma so to speak. About 18 months ago I built a large coffee table out of blackwood, I was well satisfied with the result which I finished with catalysed lacquer. The finish looked great for a while, but after 18 months of use it has become scratched and dull. I am thinking that I probably need to re-finish with polyurathene to stand the wear and while it is easy to sand back the top, it is anything but easy to properly sand the rest of the table. Hence my question: can I spray a poly finish over lacquer, or will they fight each other? What are your thoughts?

Neil

Remember: If at first you don't succeed, you're running about average.

[la Huerta]

someone will probably correct me but don't think those two go together, a solution would be to spray a few coats of shellac to seal the cat, shellac is the best at sealing anything, it forms a complete boundary for another finish...

[echnidna]

Poly will go over lacquer very well and give a far more durable top.

[martrix]

Poly will go over lacquer very well and give a far more durable top.

+1

[durwood]

There has been quite a few questions about putting different finishes over each other in various threads.

There are some easy rules to follow that may help a lot.

First avoid shellac unless its intent is to do two things:

1: seal off timber which would otherwise soak up the finish you wish to finish in.

2: use it as the finish (french polish) where you decide the use the object is being put too it is Ok to use shellac.

Shellac is a finish that differs from all the other normal alternatives. It only dissolves in metho and this makes it fast drying and easy to use providing you don't need to have a hard heat resistant finish as it hates heat, so putting hot cups or plates on it blisters it and in sunlight it also breaks down fast.

It does seal very well, as other finishes contain little or no metho, so they do not remelt the shellac and therefore dry by themselves on top of the shellac. You should avoid using it after initially putting it on the timber unless you have a real need to do so. As other finishes don't melt the shellac they also can't adhere very well so putting shellac over other coatings or between them usually results on poor adhesion and the heat sensitivity thin again.

If finishes are thinned with turps they can be put over virtually any good clean surface. The solvent does not attack others and the paints are usually naturally sticky and adhere to things well including glass.

BUT they form a coating which can easily be distroyed if a stronger solvent is applied to them such as paints with acetone, xylene etc in them. These mostly are lacquers (or 2 pack polys) so if you put lacquer onto an enamel coating it acts like paint stripper and you destroy the turps thinned finish completely and have to remove it and start again with another method.

For the lacquers sanding sealers are available to improve the finish as shellac defeats the purpose of using them as it makes it hard to get good adhesion and it is affected by heat.

If you use a 2 pack material the shellac diminishes the pluses for the same reason, it has a sealer if needed to keep the quality of the finish high.
Put it over shellac it won't stick as well as it could (same reasons as mentioned before.

You should see a pattern here.

* You can put turps based finishes over anything.
* you can put lacquer over itself or 2 pack. (as 2 pack cures and doesn't react to solvent it will not react to any solvent but being rock hard may need rubbing back to get better adhesion.)

* You can put 2 pack finishes over shellac ( but it may not adhere real well) and you can put it over lacquers (rubbing back helps ) but putting shellac in between does not help only makes the end result less durable or possibly fail completely.

It also follows that if you put down shellac and then stain or use an oil they will also be only able to dry out on top and won't adhere so they may form a poor coating or rub off unless coated but may prevent any coating sticking well as they are themselves not stuck to the shellac. Most stains and oils are meant to go onto the timber first before shellac.

A good rule is to pick a type of finish and only use products from that family for if you get it wrong all your work may fail.

Now you can alway make a wrong method work and if you think about it use the rules to get you out of a sticky situation. For instance you have a turps based varnish, don't want to remove it but do want ot paint over it with lacquer. Its going to react so thinking smart you realise that if you put shellac on first you can then lacquer.

The metho won't hirt the varnish the lacquer solvent can't get through the shellac and attack the varnish so it works. but the shellac won't stick as well as it might and the lacquer won't stick to the shellac well and as long as its out of the sun and you don't put a hot cup on the surface its probably ok.

You do have a result but you also have an unknown result it may chip easily bubble up or flake off if someone or something detrimental gets to it so its always a risk.

[Neil Hodge]

Poly will go over lacquer very well and give a far more durable top.

Thanks Bob. I think I will go with this since I don't get much fun out of finishing the same table every year or so.

Neil

[MacS]

The proper sealer in this case would be a "vinly sealer" it should always be used with the precats, it also could be used with most other coatings.

Whenever using shellac as a sealer or a barrier coat, be sure you use a "wax - free" Shellac.

MacS

[astrid]

If I can put in my 2 bobs worth
if you have gone to the trouble of making a blackwood table dont spoil it with a synthetic finish.
Blackwood takes danish oil very well if you seal it with shellac first.
then put on 2-3 or even 4 coats of danish oil
danish oil is reasonably heat and water tolerant.
I have been using it on my clients furniture for years without complaints
on of the nice things about it is that when it does get tired you can put another coat on top as long as you clean it thoughaly first and lightly buff it
no finish other than a two pack will take a pot off the stove or a superheated coffee cup
treat a finish with reasonable respect and it will last

ps dont wipe your table with spray and wipe type cleaners these are intended for plastic not wood. just use warm soapy water or just water and dont leave wet

astrid

[MacS]

Whenever, selecting a coating you need to know how much use they will be getting, its easy selecting the right coating if the pieces are basically for show, and will get very little use.

Its selecting the right coatings when the pieces get a lot of use, you need to think ahead about when the coatings do get damaged, will you be able to repair it. The best coatings for doing repairs on are the evaportive coatings.

The problem is that the more durable and chemical resistant the coatings are the more difficult they are to repair.

You need to really think about this before you select your coatings, not after you have a problem.

Think Twice, Finish Once.

MacS

http://www.macsimmons.com/

http://iswonline.com/OnlineFeatures/...of%20Finishing