Rubbing back polyurethane varnish between coats??

[woden]

Is this necessary if you do the coatings all in one go?

Thing is, this is the part of finishing that I loathe - invariably I rub a coat down and then agonise over whether it's done enough. Then I usually take into it again and overdo it - sanding through in places. I've grown up with boats and so am always having to coat something with yacht varnish. Although things were different in the past most yacht varnishs are now polyurethanes. However, I've always rubbed down (the surface, not me) between coats of this stuff and now I've just read that there's no need to if the following coat is applied within a week.

Apparently, the solvent in poly softens the recently applied coat beneath and the two layers sort of 'melt' into each other. So rubbing down is pointless provided you varnish again soon enough. Is this really the case or has some lazy charlatan propagated this idea as yet another half-baked labour saving manoeuvre? It's just that as a lazy charlatan I've a number of things that need coating and would love to bypass the rubbing down bit. But I don't want to see the varnish delaminate in about a years time.

Finally, regardless of whether this melding of coats into each other occurs in polys or not, are there any other finishes where this happens? I'm keen to know of other occasions where I can avoid the rubbing back hassle. :?

 

[woden]

Hello? Anyone?


The varnish I intend using on a tiller I'm restoring is International's Original. It's not actually a polyurethane but instead a traditional yacht varnish. But I wonder could you do what some people refer to as 'hot coating' with it as well. That is put on the next coat as soon as the previous one has dried without doing any sanding. I notice that in the data sheet they only say to sand between coats in the hint section as opposed to placing it in the method section as they do with other finishes. So maybe hot coating is feasible.

 

[Paul Chapman]

Hi Woden,

Many years ago I used to use Furniglas PU15 polyurethane varnish. I reckoned it was one of the best, but they were taken over by Evode and you can't get the stuff now. Anyway, 30-or-so years ago I varnished several kitchen implements that had wooden handles using the stuff and gave several coats at daily intervals without sanding. All these years later, and after many sessions in the washing up bowl, the varnish has remained intact. Not a scientific test, but appears to bear out what you say about not needing to sand.

Cheers :wink:

Paul

PS the nearest varnish I have been able to find to Furniglas is the stuff made by Liberon.

 

[Sgian Dubh]

Is this really the case or has some lazy charlatan propagated this idea as yet another half-baked labour saving manoeuvre?

No they haven't. Up to a point oil varnishes can be applied coat upon coat without sanding in between. As long as the previous coat is still curing the fresh coat will bond with the lower layer.

Varnish is very slow drying and a full cure takes two or three weeks. One major reason for rubbing down between coats, even when coats are applied within 24 hours of each other is to sand back debris and other objects that fall into the slow drying wet film.

If you leave a varnish coat for two or three days before applying the next coat rub down with 220 or 280 grit to provide a physical key for the next coat.

Finally, regardless of whether this melding of coats into each other occurs in polys or not, are there any other finishes where this happens? I'm keen to know of other occasions where I can avoid the rubbing back hassle.

You can easily tell if a fresh layer of polish burns into previous coats to form a contiguous whole. If the solvent for a finish dissolves cured coats of that finish then successive layers meld together.

The solvent for shellac is alcohol. Cured shellac dissolves if you wipe it down with alcohol. The solvent for the nitro-cellulose family of finishes is cellulose (lacquer) thinner. Cured coats of nitro-cellulose type finishes are dissolved by lacquer thinner. White (mineralised) spirit is the solvent for oil based varnish. White spirit does not dissolve cured oil varnish. Water, the solvent for water based varnish does not dissolve existing cured coats. Slainte.

 

[woden]

Paul, was that Furniglass stuff marketed as a yacht varnish or just exterior? You said you gave several coats at daily intervals so that suggests that you just waited until the previous coat was touch dry before putting on the next one. Is waiting 'til the next day or even later in the week too long even if the stuff doesn't fully cure for weeks?

Sgian, I'm not sure but there seems to be a contradiction in what you're saying. You say that you can apply a new coat to still curing varnish without sanding and that varnish usually takes two to three weeks to fully cure. Yet you then add that if you leave varnish for two or three days rubbing down is needed to re-coat. But surely the varnish is still a long way from cured at this point so according to what you first said rubbing back is needless. Sorry if this sounds like cross-questioning but I'm just a bit confused, that's all.

One major reason for rubbing down...
...is to sand back debris and other objects that fall into the slow drying wet film.

But when I'm coating fresh wood for marine use I'd normally go for 7 to 8 coats plus primer so the build up of earlier coats often conceals many blemishes making it only really necessary to sand back the penultimate coat to get a smooth finish.

Having said that I've still sanded back after each one up until now in the possibly mistaken belief that a key is necessary making the whole job a real chore.


A bit OT but what would the both of you sand bare wood to before applying polyurethane or yacht varnish. The former is quick thick and the latter even gloopier so I wonder could you get away with going only to 180/220 grit? Does anything finer make any extra difference?


Ps, Sgian is that slainte as Gaeilge or as Gàidhlig? :wink:

 

[Sgian Dubh]

there seems to be a contradiction in what you're saying. You say that you can apply a new coat to still curing varnish without sanding and that varnish usually takes two to three weeks to fully cure.

I did phrase it confusingly, sorry. You can recoat without sanding up to about three days after the previous coat. Intermingling- chemical bonding- of the two layers occurs in this time span. The general concensus is that after about three days the lower layer will have cured too much for a new layer to bond chemically. A mechanical key is needed after that, which is where light sanding comes in-- just a rub over to create some roughness. A full cure of oil based varnishes takes two to three weeks. You can easily tell if a finish is fully cured. If you can smell the solvents off-gassing, it's not fully cured.

...what would you sand bare wood to before applying polyurethane or yacht varnish. ...could you get away with going only to 180/220 grit? Does anything finer make any extra difference?

When sanding is followed by a film forming finish, eg, varnish, it's really a waste of time sanding coarse woods like oak finer than 180 grit. Similarly, going beyond 220 or 240 grit on fine grained woods such as sycamore and cherry is also counterproductive.

Many wood finishers seldom sand bare wood beyond 150 or 180 grit for any wood type when a film polish is the finis. Personally, I quite often go to 220 grit, but 240 grit is extremely rare. I rub down between coats of varnish, shellac and lacquer with either 320 grit or 400 grit.

In the case of oak the open grain is coarser than the 180 grit. On fine woods the film flows to fully envelope the slight furrows and high spots of the 220 grit striations. The eye can't see the striations once the polish is on.

There are plenty of published test results on this topic to support my position, but I can't provide a specific reference off the top of my head. I know the US rag Fine Woodworking had an article on the subject within the last two or three years. Slainte.

 

[Paul Chapman]

Paul, was that Furniglass stuff marketed as a yacht varnish or just exterior? You said you gave several coats at daily intervals so that suggests that you just waited until the previous coat was touch dry before putting on the next one. Is waiting 'til the next day or even later in the week too long even if the stuff doesn't fully cure for weeks?

It was not yacht varnish - just normal interior and exterior stuff. I would just wait till it felt quite dry which was normally a day. I found that I could successfully re-coat without rubbing down for about a week.

Cheers :wink:

Paul