Thread: Polyester / Polyurthene

Polyester doesn't bond very well to wood. You'd be better off using one of the finish coat epoxies designed for tabletop use.

'The first thing that is done before the polyester is applied is to apply a barrier coat (after staining and prepping of wood surfaces and metals) to ensure that the entire surface is secured. This barrier coat migrates into the wood pores and attaches itself to the wood surface. After a light scuff of the wood surface, polyester is then applied to the surface. The polyester will mechanically or chemically adhere to the barrier coat'

Wouldnt this do the trick and what expoxy do u recommend

The guys who build and refinish wooden boats avoid polyester resins like the plague, so I'd work on the theory that there is no effective way to really key poly to wood; I'd back this up with my own experiences using poly to build wood/fiberglass enclosures for car audio - i've had it shear off chipboard with little actual wood removal.

You can use something like botecote pour on gloss - Pour On Gloss : Boatcraft Pacific. , Share our passion!, or you can pick up something similar (can't remember the name, but usually in the 'craft' area) from Bunnings. (note that I've never used either of those gloss finishes - I'm more a wax and oil person)

However I think you'll find the majority of commercial furniture in Australia is finished in a two-pack polyurethane.

Thanks for the info from your experience what would you recommend the best durable finish for office furniture , partition walls etc

Two-pack polyurethane.

As poly contains an isocyanate, you'll need a proper pressurised air-fed respirator (not just a respirator mask) and spray booth setup, otherwise, at the least you/co-workers run the risk of developing an allergic reaction leading to permanently reduced lung function, or at the worst, you fall dead after your first morning's spraying.

whats the recommended number of coats and mode of application. Please note that there are wall paneling and partitioning that I am thinking of applying polyurthene coat on site as the design requires no seams between boards . My question will the poly coat be affected by the dust particles on the site or sanding after each coat will eliminate the dust .

Finally how is polyurthene with scratched will it with hold for a number of years and does it get yellow with UV in the future

Appreciate your feed back

Number of coats depends on the brand and the method of application and your desired film thickness - refer to the manufacturers data, do some samples and adjust to your requirements.

Generally, to be cost effective, polyurethanes need to be sprayed in a spray booth, otherwise the labour costs of sanding start becoming ridiculous (poly is a hard finish, after all); not to mention the very real health risks created by all the vapours drifting around a construction site, which I don't think you understand or comprehend that well. Reaction to isocyanate vapour can be anything from 'nothing at all' to 'feel really, really crap the next morning' to 'drop dead after a minute of spraying', and normal organic vapour cartridges will not filter it.

On-site application would probably be better done with an acrylic lacquer - typically these can be sprayed while wearing just an organic vapour cartridge in your respirator, they flash off in minutes, reducing the amount of time that dust and insects have to stick on it, and they compound well, enabling you to get a good high gloss by buffing the final coat. They are also easier to retouch to remove scratches.

No seams between boards for wall paneling is not an achievable goal; it is possible have it seamless at a point in time, but natural wood movement (yes, even in MDF) will open up the seams eventually. Depending on local climate, 'eventually' could be in the first few months after the air conditioning is turned on, or not for some years. Far better to incorporate a design element that will allow individual panels to move slightly. Movement in wooden floors will happily push a brick wall down, if no gap is left to accommodate expansion.

Poly is a very durable finish and will typically retain that piano gloss finish for a few years as a tabletop, but scratches will mount up. My thoughts on poly as a tabletop - it looks really good for 1-2 years, ok for 3-4, but after 5-6 it starts looking like it needs to be redone, which requires sanding back the whole thing to bare wood and respraying.

Poly will be affected by UV, but not as much as the wood underneath. The wood will probably fade from UV long before the poly picks up any significant discolouration.

Polyester resins are also somewhat more temperature sensitive than Polyurethane resins and can delaminate or otherwise fail if exposed to Australian temperatures - I have several bows that came to pieces one summer as the manufacturer had used -ester resins to save a few cents per item. All Poly-whatever resins are quite critical in their mix ratio for optimum performance - they are working to make them more tolerant of off mixes - and some are also temperature & moisture sensitive while curing.

Which is a long way of saying 'I agree with Master Splinter'

Originally Posted by Master Splinter

Number of coats depends on the brand and the method of application and your desired film thickness - refer to the manufacturers data, do some samples and adjust to your requirements.

Generally, to be cost effective, polyurethanes need to be sprayed in a spray booth, otherwise the labour costs of sanding start becoming ridiculous (poly is a hard finish, after all); not to mention the very real health risks created by all the vapours drifting around a construction site, which I don't think you understand or comprehend that well. Reaction to isocyanate vapour can be anything from 'nothing at all' to 'feel really, really crap the next morning' to 'drop dead after a minute of spraying', and normal organic vapour cartridges will not filter it.

On-site application would probably be better done with an acrylic lacquer - typically these can be sprayed while wearing just an organic vapour cartridge in your respirator, they flash off in minutes, reducing the amount of time that dust and insects have to stick on it, and they compound well, enabling you to get a good high gloss by buffing the final coat. They are also easier to retouch to remove scratches.

No seams between boards for wall paneling is not an achievable goal; it is possible have it seamless at a point in time, but natural wood movement (yes, even in MDF) will open up the seams eventually. Depending on local climate, 'eventually' could be in the first few months after the air conditioning is turned on, or not for some years. Far better to incorporate a design element that will allow individual panels to move slightly. Movement in wooden floors will happily push a brick wall down, if no gap is left to accommodate expansion.

Poly is a very durable finish and will typically retain that piano gloss finish for a few years as a tabletop, but scratches will mount up. My thoughts on poly as a tabletop - it looks really good for 1-2 years, ok for 3-4, but after 5-6 it starts looking like it needs to be redone, which requires sanding back the whole thing to bare wood and respraying.

Poly will be affected by UV, but not as much as the wood underneath. The wood will probably fade from UV long before the poly picks up any significant discolouration.

I really appreciate your feedback and advice

Regarding the seams I have attached panels that will go on to the wall and they are larger than the MDF boards .Thus they have to be installed and painted and cleared on the wall to go the desired effect . Your advice would be appreciated

Not sure what you are trying to achieve - if you clearcoat a matt finish, you'll end up with a gloss finish.

If the client wants white, why not use a satin finish melamine faced board - much, much more durable than any paint finish and (at about $35 odd for a 1200x2400 board here) it's way cheaper than a paint finish, too.

Originally Posted by Master Splinter

Not sure what you are trying to achieve - if you clearcoat a matt finish, you'll end up with a gloss finish.

If the client wants white, why not use a satin finish melamine faced board - much, much more durable than any paint finish and (at about $35 odd for a 1200x2400 board here) it's way cheaper than a paint finish, too.

The pattern is on MDF 18 mm and the depth of the panel which will be done on CNC machine will be 5 mm deep . If I use melamine then the patern which is 5mm will need to be painted

In that case, you are back to polyurethane.

Go from the CNC place to the nearest spraypainters and they can spray the routed panels with polyurethane in a spray booth. You don't want to waste time even thinking of sanding the CNC'd areas after they are painted; Poly gives a good gloss right off the spraygun; if it has to be a matt finish, talk to the spraypainters about what they recommend as many matt finishes are a maintenance nightmare as they mark too easily.