Why does the rubbed surface do this?

[Tegmark]

Using canned poly. I noticed after I successfully rubbed the previous coat & washed it. Later what looked like a spec of dust, but it wouldn't come off at all. I had to rub it out with more coarser sand paper, the surface seemed brittle. So after all this why do this surface problems happen?

[Master Splinter]

That's the nature of polyurethane.

At its essence, it is a layer of plastic on top of the wood. As it has fairly good solvent resistance, it's not going to dissolve in its own solvent (unlike, say an enamel finish) so any keying to previous finish layers has to be done mechanically, and thin layers are always going to be brittle.

[Tegmark]

That's the nature of polyurethane.

At its essence, it is a layer of plastic on top of the wood. As it has fairly good solvent resistance, it's not going to dissolve in its own solvent (unlike, say an enamel finish) so any keying to previous finish layers has to be done mechanically, and thin layers are always going to be brittle.

To be honest I don't understand this. When you say nature of polyurethane does this mean its just unreliable & shouldn't be used? I gotta say its a hit & miss & I hate this stuff. This was a thick layer. The flaking where you can chip at it with finger nail appears later.

[Master Splinter]

Polyurethane should be recoated before it cures, which is usually within 6 -24 hours for best intercoat adhesion. And don't sand coats finer than about 180 grit when recoating, again for adhesion.

That's because longer than ~24 hours and it's not going to bond chemically with the previous layer, and if you sand too fine the poly may tend to bridge the sanding scratches instead of wicking into them. Thoroughly dedustng after sanding is also needed.

Ideally, follow the manufacturer's instructions to the letter to give you a baseline to work from, and fiddle with the technique from there.

Old poly that's been in the shed for several years may also give less than serviceable results.

Personally I prefer either tung oil or clear acrylic car lacquer.

[Evanism]

Poly can be a tricky beast. I have a big can of Cabots, but it is rarely used now, but I did use it extensively.

The surface does need light sanding between coats. Temperature needs to be within guidelines and nibs (little specs of dust) are a headache. My personal experience is I use a clean 150 sheet of paper (well, a festool disk and the removable pad from the sander to hold it!) and give it a good abrading.

It can't be done too early as its still not set, but too late is a nightmare. It hardens to a rock and I find it a very hard job once that occurs. De-dusting with a $1 tack cloth from the auto store is imperative as is a dust free environment. Once the job is finished, wait a few days for it to be very cured, then rub it back with 0000 steel wool (Bunnings and Masters sell it). This last step removes nibs and other tiny defects.

I agree with MasterSplinter, pre-catalysed lacquer is awesome ($80 at The Paint Place), it sprays beautifully, flat and dries very fast. Lots of light coats over a 1 hour period and it's magic.

[Evanism]

If you do happen to get little bubbles, grab a small butane torch and wave it over the surface. They pop and extrude themselves leaving everything very flat.

Obviously you use only the lightest of touches/waves!

[Tegmark]

Polyurethane should be recoated before it cures, which is usually within 6 -24 hours for best intercoat adhesion. And don't sand coats finer than about 180 grit when recoating, again for adhesion.

That's because longer than ~24 hours and it's not going to bond chemically with the previous layer, and if you sand too fine the poly may tend to bridge the sanding scratches instead of wicking into them. Thoroughly dedustng after sanding is also needed.

Ideally, follow the manufacturer's instructions to the letter to give you a baseline to work from, and fiddle with the technique from there.

Old poly that's been in the shed for several years may also give less than serviceable results.

Personally I prefer either tung oil or clear acrylic car lacquer.

Thanks, appreciate the detailed advice. Shows I'm still learning. But in all fairness this stuff is as reliable as a psychic. Sometimes great, others devastating. I usually sand with 400 as any lower grit might leave visible scratches.

[Master Splinter]

Unfortunately, scratches are what are needed to ensure that the new layer can adhere once the poly is out of its green stage and is no longer chemically reactive...

...well, adhere is the wrong word...think of it as pouring jelly into a container - if the container is nice and smooth and has a bit of draw, then the jelly will pop out nicely when set; that's like a finely sanded surface, and, like the jelly, there's nothing holding the poly in - there's no real chemical bonds.

If you pour the jelly into a container that's been lined with really crinkly aluminium foil, it's not going to pop out nicely when set, and when you do finally separate the two, you'll probably make a big mess and have bits of jelly still stuck to the foil in all the crinkly bits; that's a coarse sanded finish.

Intercoat sanding is done purely to remove dust nibs/mosquito feet/cat paw marks and prep the surface for the next coat.