Making a neat job with foaming polyurethane adhesive

[Dan Smithers]

Hi,

I just tried out foaming polyurethane adhesive for the first time and wondered whether there are any tricks for making a neat job? It's the sort that goes on as a brown treacle-like goo and foams up after a few minutes.

I applied liberally and then spent the next 20 minutes wiping it off and still had a lot to trim away the next day. (note to self: I still owe my wife a new bottle of nail varnish remover).

It didn't seem to be very sandable, or was I just not enthusiastic enough?

Perhaps my problem is that I want it to fill the gaps in the joint but leave a clean edge on the exposed outside.

thanks

dan

[Dave Plowman (News)]

IIRC, you'd best not leave it exposed to the outside - it deteriorates rapidly in sunshine or maybe even just light. Cut it back slightly and cover with a cement fillet.

[RubberBiker]

First off, disposable gloves are essential.

Polyurethane glue works best in closely fitting, and firmly clamped timber joints. Whilst it will gap-fill, the foam has no strength. It's pluses are the strength of the joint where it is well compressed, and that a good joint is truly waterproof.

Experience will tell you how much to apply. On a good tight joint, you will get some foam expelled, but you can take that off with a scraper once hard, and then sand or plane the face - but I would emphasise the good, tight joint.

What sort of work are you doing wit it?

[Dan Smithers]

Ummm, yes.

That's the bit I was missing. I slopped it around liberally and was then horrified as it expanded. A typical first time use mistake I guess.

I am repairing a wooden dinghy - pin and gluing a batten to strengthen a repair. The underneath board is not completely flat so something that fills the gaps is useful.

I did follow the instructions that said to remove excess with acetone but in the end I did as you suggest and trimmed once hard. Next time I'll try spreading it more thinly - a thin smear over the mating surfaces and a little more in the gaps.

thanks

dan

[TheOldFellow]

I think for a boat repair you would do better with a two part epoxy, used with a filler to gap fill. It will be interesting to find out in a year or so how the polyurethane glue worked out in the long run.

I use poly quite a bit, I find you don't need more than a smear on one surface (wet the other) for a well-fitting joint. I never wipe the wet glue-foam up, it's better to take it off with a sharp chisel when it's set.

R.

[george (dicegeorge)]

would masking tape help?

[g]

[RubberBiker]

West Systems epoxy would be the most widely used product for glue/ filler type repairs on wooden boats.

[Steve Firth]

Foaming polyurethane glue is a standard glue for boatbuilding. I think the only problem this time around was the user's inexperience. PU glue foams much more than one would expect and it only needs a light smear of glue, as you observed.

[Dan Smithers]

Thanks Steve, I had been recommended this by several people who repair wooden boats.

You're quite right, it is mainly my inexperience. Can I use a spreader to make a thin layer (obviously disposable) or does this upset the adhesive?

[TheOldFellow]

You live and learn, thanks for the info. I'll have more faith in it outside now. R.

[Rob G]

I don't know anything about boat building, but I got a gunnable tube of polurethane glue recently, and apart from the necessity for gloves and making sure that the end of the tube is well sealed after each use, I'm in all favour of it.

It's the 5 minute set type which again may not be the boat building variety, but having had the brown goo type in a tin, the gunnable type is so much easier to apply and doesn't go off in the tin.

One asset I didn't realise is that it will glue more than just wood; I haven't tried on on stone or concrete yet, but I have glue aluminium to aluminium and Al. to wood.

I would go with the cleaning up afterwards - meths works for cleaning it when wet and can be got in 5l containers so is a bit cheaper than nail varnish cleaner!

Rob

[Steve Firth]

I think it's the best of the one-shot glues available, it takes the place of the older phenolic/casein glues.

Eyeballing how much to use comes with practice. It's a good idea to play with glueing scrap first to get a feel for it.

Yes, that's fine but you have to work fast. The glue starts to foam very quickly.

Two-part epoxy filled with glass microballoon fillers as mentioned by TheOldFellow is also good but it takes a different type of skill and it also works out much more expensive IMO.

One trick you can use is wax to coat the bits you don't want to stick, but this obviously causes problems later if you decide you want to glue something to a part you have waxed. Another is to use masking tape (preferably blue) to mask off bits of wood you don't want to have to trim foam from.

[Steve Firth]

You're welcome, just remember that it's not UV stable. So you need to cover it with paint or varnish if it is exposed to sunlight. Oddly enough although polyurethane glue goes brown in sunlight two-pack polyurethane varnish is stable and remains transparent. I've used two-pack polyurethane over both expoxy and PU glue and it stops damage by sunlight - I've got pieces that are ten years old and have been in salt spray and sun all of their life.