Polyurethane not finishing well.

[icor1031]

I use the high gloss polyurethane, and this is what happens when it's all dried.
What's going on? I've re-tried it 2 or 3 times, and this keeps happening.

[shedbound]

is it brushed, wiped on or sprayed? could be moisture or dust/lint

[icor1031]

It is brushed on.

I cleaned the table with mineral spirits and a rag before I put that layer on.

[Master Splinter]

Preparation method/sandpaper grit? Number of coats? Sanding between coats?


In my personal opinion, the best finishing schedule for polyurethane involves sanding back to bare timber, then sanding and buffing the wood with an oil finish to 1200 grit and buffing the final finish to a good luster.

During this process, the poly varnish can is used as an elevated stand for your coffee cup to minimize the amount of sanding muck it collects, and the weight of a full or nearly full can makes sure it will be hard to knock over accidentally. At the end of the finishing process you simply throw the poly varnish can out!

[icor1031]

It's MDF, mate. No displaying bare wood.

I sanded black paint to 220.
Put on a coat of poly (this time it was semi, last time it was full gloss.), and sanded it at 800.
Cleaned with mineral spirits and rag.
Applied coat of full gloss - and this was the result.

Preparation method/sandpaper grit? Number of coats? Sanding between coats?


In my personal opinion, the best finishing schedule for polyurethane involves sanding back to bare timber, then sanding and buffing the wood with an oil finish to 1200 grit and buffing the final finish to a good luster.

During this process, the poly varnish can is used as an elevated stand for your coffee cup to minimize the amount of sanding muck it collects, and the weight of a full or nearly full can makes sure it will be hard to knock over accidentally. At the end of the finishing process you simply throw the poly varnish can out!

[icor1031]

Is it because a lot of my polyurethane turned hard in the can? I assumed because it seemed to turn back to liquid that there wouldn't be a problem, but did it create these specks?

[Master Splinter]

Sand MDF to 120 grit, apply primer, sand to 120, apply black paint, sand to 180, apply black paint, sand to 180, apply black paint, sand to 180, apply black paint, sand 240 ..... to 1200 and then use a car paint buffing compound to bring a high gloss to the finish.

Then throw the lumpy poly into the bin. (and a turps wipedown may not be the best thing if it was a water based poly, as turps leaves a slightly oily residue)

An acrylic enamel would be a better paint finish (the stuff used for painting cars) for MDF.

More coats may be needed depending on your brushwork...the auto paint has the advantage of coming in spray cans.

[A Duke]

Did you by any chance use hair conditioner on your brushes the last time you used them?
Regards

[wheelinround]

As your in South Dakota what was temp at the time? Moisture content in the air and ambient temp in the shed all these are factors effecting finish.

The possibility of the solvent you were using to wipe over between coats could also be the problem.

To attain a high gloss look with a base coat such as your black I use a different method.

I use a base coat of plain colour (black) to apply , sand, then mix 50/50 of high gloss and black and apply allow to dry, sand, mix a 25% Black and 75% high gloss thinned 2 to 1 and apply.

Just make sure both gloss and base coats are the same types of compounds ie enamel.

I did this with a cabinet we are about to get rid of after 25 years and it still has minimal scratches to the surface it was a TV cabinet.

[icor1031]

Wheel,

It was around 80º in the room.

What do you mean by this?
"The possibility of the solvent you were using to wipe over between coats could also be the problem."

[icor1031]

No, only brush cleaner..?

Did you by any chance use hair conditioner on your brushes the last time you used them?
Regards

[A Duke]

No, only brush cleaner..?

Good.
Applying a bit of hair restorer to a brush after cleaning keeps it nice and soft and it does not get that crisp feel BUT make sure it does not contain silicon or it causes "fish eye' bubbles in the finish. While thinking of silicon you didn't use any thing like WD40 around it did you?
Just a possibility when looking for reasons.
Regards

[icor1031]

I didn't.
Thanks.

Good.
Applying a bit of hair restorer to a brush after cleaning keeps it nice and soft and it does not get that crisp feel BUT make sure it does not contain silicon or it causes "fish eye' bubbles in the finish. While thinking of silicon you didn't use any thing like WD40 around it did you?
Just a possibility when looking for reasons.
Regards

[wheelinround]

Wheel,

It was around 80º in the room.

What do you mean by this?
"The possibility of the solvent you were using to wipe over between coats could also be the problem."

Had to convert 26C or 80F isn't to bad but what of ambient temp and moisture in the air.

You didn't mention if you sprayed or hand applied.??

If sprayed had you emptied/drained the tank does it have a moisture filter, even sweating over the top and fine droplets can cause similar.

Out of the blue thought is the roof or ceiling sealed or just a tin/tile roof and could a breeze have been about?

Wiping a solvent over the top prior refinishing

Cleaned with mineral spirits and rag.

a tack rag is all you need to remove dust particles a solvent can eat into the prior coat.

So many possibilities