Can't get 2Pack Polyurethane to level out properly

[Tegmark]

Here is a test piece of Hard wood that is pretty flat in surface shape & has two coats of 7008 by brush over stain.



1st coat was thinned for sealing. The photo shows the second coat that was moderately thick litley sanded. Anymore sanding I will start to rub through into stain. As you can see all around the border close to the edge of wood there is a rise in the clear & a run off right near the edge where its thinner. It's very hard to sand these out without going into the stain. The coats are not that thick at all to have this happen. Using oil based before this never happened even if the coat was very thick. It happens with a few different brushes no mater how perfect the brushing is. What I wan't to know is if this is what all 2pack Polyurethanes do or just 7008? As it is for floor boards mostly after all.

If this is just part of 2pack, how should I go about this? If I litley sand each coat I think the overall shape will just continue & even amplify & when I sand the final coat I will get witness lines I bet.

[Arron]

I have just had a look at a vanity top that I did with 7008 a few years ago and there isn't any buildup near the edge - neither buildup nor thinning. I think the sanding done on this job was minimal so no, a buildup/thinning on the edge is not a feature of the product. I think it's occurring due to some aspect of your application.

Looking at your job, it seems the finish is partially dissolving the stain. I don't use stain often enough to comment on this.

[soundman]

Teg mate ..... if you are trying to brush pretty much anything unthinned .... it aint going to flow out.
The other issue a great many tend to ignore with two pack product is the whole temperature and humidity thing ...... any body half serious about finishing has a thermometer and a hygrometer in their shed.
Some of the two packs and epoxies get nearly unmanagable outside their optimum temperature and humidity window.

cheers

[Tegmark]

I have just had a look at a vanity top that I did with 7008 a few years ago and there isn't any buildup near the edge - neither buildup nor thinning. I think the sanding done on this job was minimal so no, a buildup/thinning on the edge is not a feature of the product. I think it's occurring due to some aspect of your application.

Looking at your job, it seems the finish is partially dissolving the stain. I don't use stain often enough to comment on this.

The stain is as far as I'm aware fine. Not sure about dissolving?

cheers

[Tegmark]

Teg mate ..... if you are trying to brush pretty much anything unthinned .... it aint going to flow out.
The other issue a great many tend to ignore with two pack product is the whole temperature and humidity thing ...... any body half serious about finishing has a thermometer and a hygrometer in their shed.
Some of the two packs and epoxies get nearly unmanagable outside their optimum temperature and humidity window.

cheers

I did many oil based pieces thick as heck & this problem never happened. Any thinner a coat & will have to apply so many to get depth.
I don't see the need for a thermometer & hygrometer. The tech specs show a good range & I have applied this 2pack at 12C in shed at night & 27C in warm house & the results are the same usually.

Imo ~18-20C is best to avoid it setting to quick which is a problem on large areas with a large brush.

[Tegmark]

Well this must be a problem with 2pack & caused by some surface tension effect. I tried to apply a very thin coat by brush & it still happens. I think you need it spray gun thin to prevent this.

With thin coats sanding out small pinholes & the unavoidable bulges/rises right near wood's edge leads to a high chance of rubbing into stain & also causing witness lines.
If bulges are not sanded out they can slightly show under next coat, & also with only lite sanding the overall pattern of the surface just repeats. It seems pretty obvious a thick coat is vastly better than a thin coat.

So my method now is first 3 coats thin & lite sanding, from 4th coat apply very thick with aggressive sanding on flat block & finish at 5-6coats.