Thread: Need a lesson/help with sanding and finishing

If you want a high gloss finish with WOP, you need to build the coats up. WOP goes on very thinly and a couple of coats won't hide the scratch marks you make with sanding and with steel wool.

Remember also that hand sanding can leave scratch marks more than a decent ROS so I would try to go higher in the the grit range before applying WOP.

I've never used gloss Wipe-on, only satin and never really used steel wool.

After sanding the raw timber using 240 then 360 sand paper (i like the white non-clog paper) I apply 3 or 4 coats of wipe-on, sanding between coats using 360.

After the piece has enough coats to seal it I move to wet and dry paper. Used wet (the reason it needs to be sealed), i move my way through the grits between each successive coat untill i reach 1200 (ie: 400,600,800,1200). Then keep applying coats and sanding with 1200 untill i'm happy with the build up and finish.

I can use up to 8 to even 10 coats of wipe-on.

Because i use satin i then use EEE ultrashine and Trad wax to give a nice smooth sheen. Not sure if this whole process will work with gloss though.

Hope this helps.

Steven.

OK, thanks for the tips. I'll go back to the start and redo the sanding. Up to higher grades.
I'll leave it for a while and try some wet and dry.
Frustrating that I can't see the scratches until after the WOP has dried.

Any way that I might be able to see them before applying the WOP or the next coat? That way I could rectify them.
The job seems smooth as before the coat of WOP.

One problem that consistently comes up time after time, is peoples thinking that they have to sand finer than the realy need to and at the same time failing to properly sand out the scrtches from the previous gritt.

Of course none of this shows up till the final layers of finish.

This can be particularly bad in softer timbers...silky oak is a terror for it.

Unless you are talking fine rubbed finishes like french polishing and some oiled finishes there is very little advantage going past 180 or 240 before the final coats of finish.

As for steel wool.....it is of very little use for preparation, because it cuts very poorly, that is why it is used for rubbing out finishes...it should not be used on raw wood or preparation coats.

The other thing is using some sort of sanding sealer ( shelac is fine) between grits, particularly on soft timbers....it causes the grain and fur to stand up to be cut cleanly AND the next coat shows up the flaws in the previous sand.

I'd be starting way back at the early grits like 120 and make sure you have all the previous marks and scratches from the previous grit worked out before you move onto the next and don't miss any gritts....several goes with sealer and the same grit may be required if the previous step is a bit lumpy.

recoat with thinned shelac after every sanding ( and let it dry off properly) and don't move up a gritt till the marks from the previous are gone.


Remember you may fill scratches ( quite poorly actually)with a clear product but they will still show in the finish thru many layers above.

leave the steel wool alone and only use it for rubbing out top coats and usually with oil or wax.

Make sure you get your 180 gritt sand right with shealc, you may find you wont need to go to 240 gritt till you have some top coats on and you may go 240 between several top coats.

go too fine and you will serioulsy compromise you adheson between coats.

cheers

Thanks for that great description Soundman.
Brought up a couple of things I had been doing wrong.
I'll work on it again.
When I get it right I'll post another picture.
Lyle.

I'm not sure that the polyurethane would have any trouble getting sufficient grip if you used finer grits in between coats of WOP.

I can understand the problem if there was no sanding done at all on a hardened gloss surface, but once that is broken the scratch marks (of very fine grit), do in fact hold sufficiently. Particularly if you are doing several coats in the same day.

I have never used the method of one coat poly, one coat shellac and so on, but I have successfully finished a number of pieces with high gloss Poly finish ( using WOP) using very fine grits between the final coats.

Lyle mate, do ya self a very big favor and buy a coppy of "the polisher's handbook" by and from the owner of this forum.

I am sure you will find a u beaut link on here somewhere and many of the Australian woodwork shops carry it.

It covers most of these isues.

TP1, polyeurathane especially is prone to poor adhesion particularly if the undercoats have fully cured out.

Polyeurathane goes thru a multistage curing process......without going into too much detail, if you overcoat prior to the polly going off hard it will partly disolve the underlayer and produce a very strong chemical bond, regardless of sanding or not...for standard polly that is about 48 hours depenging on weather.

Once polly has cured past that stage it is wise to sand thoroughly if you expect a reliable bond to the next coat...and no finer than 180 or 240

It is a very common mistake to sand too finely under and between finishes.

Fine wood working did a series of tests on the matter, that incluided detailed surface examinations and adhesion tests on a number of finishes.

Their results were conclusive....very little point going much past 180 gritt under and between film formimg finsihes.

lots of people sand to very fine grits under and between finishes.....mostly it is simply a waste of time in some cases it can lead to finish failure.

the very fine grits are generally of use after the last top coat for flattening and rubbing out the finish.

Even when I am doing pens, which i have not done for a while.....If it is wood and I am using shelawax, shealc or even CA I do not sand past 240 gritt till I have the last coat of finish on, then I will run up to 600 and some sort of compound EEE ulra shine or something else.

cheers

Originally Posted by soundman

.

TP1, polyeurathane especially is prone to poor adhesion particularly if the undercoats have fully cured out.

Polyeurathane goes thru a multistage curing process......without going into too much detail, if you overcoat prior to the polly going off hard it will partly disolve the underlayer and produce a very strong chemical bond, regardless of sanding or not...for standard polly that is about 48 hours depenging on weather.

Once polly has cured past that stage it is wise to sand thoroughly if you expect a reliable bond to the next coat...and no finer than 180 or 240

I was referring to building up coats of poly on a new build as this is what I understood the original question referred to. In my case, I will do several coats of WOP in the same day. Some sanding maybe necessary between coats particularly for dust nibs etc. If you use 240/320 grit this purpose when you want to ultimately have a piano gloss finish, scratches will show through the next couple of layers.

I'm glad you referred to the 48 hour window, because sanding between coats during that time should be done primarily for surface levelling, not necessarily polyurethane grip. With this in mind, when approaching the final layers of WOP coats, I will usually start the sanding using an ROS on mid speed with 800 grit, then working up to 2000 ( or higher if necessary), to avoid scratches showing through. I have successfully done dining tables and coffee tables with this method and have polished the surface ( after curing) with no scratches showing through.

I think its also important to point out the difference between sanding bare wood and sanding a polyurethane finish which is less than 48 hours old. Using 240 grit on the bare wood does not lead to scratches showing through. Its only when it is used between layers of high gloss polyurethane do the scratch marks show.

To put it bluntly if you are trying to achieve a fully flat high gloss finish with normal Polyeurathane you are working a fools erand, there are a lot of other products and methods that will achieve the result with a lot less work.

If 240 grit is showing scratches thru the next coat you are not laying up insufficient film thickness, this will result in two things, there will be insufficient solvent in the new coat to reflow the scratches in the previous coat and there will be insufficient dried film thickness to fill them.

There is a very good argument if you are thinnig polly far enough to get a film that thin you are probably past the point where the resin strength is compromised. Pollyeurathane is a fairly coarse resin and requires some bulk to perform to specification, unlike the drying finishes like shealc which are fine resins and will tolerate being divided heavily with solvent.


Also I question the wisdom of trying to apply more than a single coat of normal polly in one day it simply will not cure fast enough to sand reliably.

Laying up very thin coats of normal polly simply flies in the face of the nature & limitations of the product.

This is why things like nitro and acrillic laquers are so popular...it is possible to lay up 6 properly cured, good film thickness coats in a favorable day.
Even more when you watch obviuos scratches in the previous coat simply melt away, as long as the previous coat has no sanding errors or dust nibs a full gloss final coat will self level and a fully flat gloss finish will result.

Even normal polly will self level and flow out reasonably well if you allow enough film thickness, that is what it is intended to do.

If you want to lay up very thin coats and do a lot of sanding and rubbing, there are much better products than normal polly, and then you are getting in the the relm of rubbed finishes.

Consider that P240 paper has a gritt of approximately 7 microns or .007mm
The recomended dried film thickness of standard Estipol is 25 microns.

You might be happy doing what you do , but it isnt normal practice and certainly is not the most efficient way of doing it.

cheers

I don't think anyone doubts that lacquer would be preferable for high gloss finishes but on this forum some of us hobbyists aren't set up for spraying lacquer. Something I'd like to do in future, however.

But, Wipe on Poly can in fact be applied in multiple coats in one day as it is designed to be done ( according to its American manufacturer). It is more than just diluted polyurethane - I have tried thinning full strength poly varnish and do not get the same results.

Fair enough but comming back to the original post.

even going to 320 and 360 is not a hell of a lot finer than 180.......360 is only half the grain size of 180.......I regularly hear of people sanding 600 and higher between coats and as with the original poster sanding bare untreated wood with very fine grits and steel wool.


back to the wipe on finishes, which are more like rubbed finishes.

By far the best approach is to build up a fully choked and properly sanded base using shelac or some other compatable sanding sealer and getting that properly sealed, filled, flatened and sanded to 180 gritt and then working up whatever it is with far less coats and far less effort.
It may bee if you are using a rubbed finish that you would take the last sealer coat finer.

One of the biggest revalations to me was doing most of the work in the sealer stage and with coarser grits than trying to do it in the later coats and the finer grits and that is regardless of product.

If you are finding nibs or scratches comming thru from previous coats the preparation has been inadequate

lots of people will do only 1 coat of sanding sealer and then many coats of top coat....I will generall tend to do as many coats of sanding sealer as needed and then very few coats of top coat often only one or two.

cheers