Should I use mineral spirits after sanding between polyurethane coats

[sandy...@hp.com]

The first application of polyurethane finally dried on my red oak
baseboard. I sanded the baseboard and used a tacky cloth to remove the
dust. Should I wipe down the baseboard with a rag dipped in mineral
spirits before I apply the next coat of polyurethane?

Thanks.
Sandy

[Bewildered]

<sandy...@hp.com> wrote in message
news:1169062977....@38g2000cwa.googlegroups.com...

Why would you want to do that?
I certainly never have.

[DJ Delorie]

No. Poly uses a mechanical bond between coats, you want as little as
possible between coats so as to get the best bond. Just make sure you
get the dust off, that's all.

And I hope by "finally dried" you mean "I waited as long as the can
said to wait". If you don't recoat in the time specified, you need to
wait at least a week or two (to fully cure) and sand before recoating.
If you recoat in the time specified, you only need a light sanding (to
remove raised grain and such) as the poly hasn't fully cured so it
will bond with the next layer whether you sand or not.

[Stoutman]

>> The first application of polyurethane finally dried on my red oak
>> baseboard. I sanded the baseboard and used a tacky cloth to remove the
>> dust. Should I wipe down the baseboard with a rag dipped in mineral
>> spirits before I apply the next coat of polyurethane?
>
> No. Poly uses a mechanical bond between coats, you want as little as
> possible between coats so as to get the best bond. Just make sure you
> get the dust off, that's all.

Once cured there is nothing wrong with wiping down with mineral spirits. I
have done this with as little as 8 hours curing time.

>
> And I hope by "finally dried" you mean "I waited as long as the can
> said to wait". If you don't recoat in the time specified, you need to
> wait at least a week or two (to fully cure) and sand before recoating.

B.S. You can sand after 24hrs. I usually use 320 grit wet/dry paper and
lightly sand.

> If you recoat in the time specified, you only need a light sanding (to
> remove raised grain and such) as the poly hasn't fully cured so it
> will bond with the next layer whether you sand or not.

So are you saying that you can only sand during the time specified on can (3
hrs usually) and after one week? This makes no sense.

[Stoutman]

Sure. There is nothing wrong with doing this. As long as you wait long
enough for the poly to cure sufficiently (varies by temperature/humidity).

If you were able to sand successfully, than your poly was cured
sufficiently. There is no danger in wiping sufficiently cured poly with
mineral spirits. I have done this NUMEROUS times.

After all, most people use mineral spirits as a solvent to dilute
polyurethane. How could this same solvent be counter indicated for wiping
cured poly? :)

Go right ahead!

Stoutman,
www.garagewoodworks.com

>
> Thanks.
> Sandy
>

[Stoutman]

>> The first application of polyurethane finally dried on my red oak
>> baseboard. I sanded the baseboard and used a tacky cloth to remove the
>> dust. Should I wipe down the baseboard with a rag dipped in mineral
>> spirits before I apply the next coat of polyurethane?
>>
> Why would you want to do that?

The mineral spirits will remove any residue left behind by the tack cloth.
Nothing wrong with doing this if the poly has cured sufficiently.

> I certainly never have.

Therefore it's not rational?

>
>

[cse...@mts.net]

>After all, most people use mineral spirits as a solvent to dilute
>polyurethane. How could this same solvent be counter indicated for wiping
>cured poly? :)
>
>Go right ahead!
>
>Stoutman,
>www.garagewoodworks.com

Then wipe it down with alcohol to get rid of the solvent reside.

Pete

[DJ Delorie]

"Stoutman" <.@.> writes:
> Once cured there is nothing wrong with wiping down with mineral
> spirits. I have done this with as little as 8 hours curing time.

Cure time depends on the poly. The stuff I use isn't cured until at
least a week passes. After 8 hours it's only dry, but not cured.

> > And I hope by "finally dried" you mean "I waited as long as the can
> > said to wait". If you don't recoat in the time specified, you need to
> > wait at least a week or two (to fully cure) and sand before recoating.
>
> B.S. You can sand after 24hrs. I usually use 320 grit wet/dry
> paper and lightly sand.

Well, nobody is going to stop you if you want to sand whenever you
feel like it. I'm assuming the OP wants as good a finish as is
practical.

I've noticed that before the recoat time, I can sand lightly, and only
to remove raised grain. The poly is still soft enough that if I try
to sand into it, it just rolls up into lumps (like a pencil eraser).
Only after it's fully cured is it hard enough to actually sand into
the poly.

When fully cured, sanding poly should result in a fine, light, dusty
powder. Like what you usually see with sanding sealer.

> So are you saying that you can only sand during the time specified
> on can (3 hrs usually) and after one week? This makes no sense.

I'm saying that if you recoat outside of the recommended times, the
coat won't hold as well. Why? Because of how the poly layers join.
During the cure time, the poly molecules go from loose to tight, so if
you recoat during that time you can still hook into the uncured
molecules for a mechanical join at the molecular level. Once they're
cured too much, this is no longer an option for a reliable join, so
you wait until it's cured enough (i.e. hard) to sand it to make a
mechanical joint at the macro level.

[Stoutman]

> Cure time depends on the poly. The stuff I use isn't cured until at
> least a week passes. After 8 hours it's only dry, but not cured.

The curing process starts after it is applied. There are degrees of
'cured'.

>> B.S. You can sand after 24hrs. I usually use 320 grit wet/dry
>> paper and lightly sand.
>
> Well, nobody is going to stop you if you want to sand whenever you
> feel like it. I'm assuming the OP wants as good a finish as is
> practical.

24 hrs is plenty of time to lightly sand with 320 wet dry.

> I've noticed that before the recoat time, I can sand lightly, and only
> to remove raised grain.

What do you think I am talking about here? Re-read what I wrote above.

>The poly is still soft enough that if I try
> to sand into it, it just rolls up into lumps (like a pencil eraser).

When it behaves like this during sanding your poly has not 'sufficiently'
cured. Granted curing time is governed by humidity and temperature.

> Only after it's fully cured is it hard enough to actually sand into
> the poly.

Wrong. It does NOT need to be "fully cured", only sufficiently cured.

> When fully cured, sanding poly should result in a fine, light, dusty
> powder. Like what you usually see with sanding sealer.

I get the same results when 'sufficiently' cured. Doesn't have to be "fully
cured".

>
>> So are you saying that you can only sand during the time specified
>> on can (3 hrs usually) and after one week? This makes no sense.
>
> I'm saying that if you recoat outside of the recommended times, the
> coat won't hold as well. Why? Because of how the poly layers join.
> During the cure time, the poly molecules go from loose to tight, so if
> you recoat during that time you can still hook into the uncured
> molecules for a mechanical join at the molecular level. Once they're
> cured too much, this is no longer an option for a reliable join, so
> you wait until it's cured enough (i.e. hard) to sand it to make a
> mechanical joint at the macro level.

Sanding is usually required after 24 hrs.

[Jill]

Feisty woodworkers up in here! 😄

--
For full context, visit https://www.homeownershub.com/woodworking/should-i-use-mineral-spirits-after-sanding-between-polyureth-360422-.htm

[Bob Davis]

Ha, good comment, Jill.

I am doing every thing wrong. I use a water base poly which dries in a hurry. I sand 2-3 hours after first coat with 320. It sands creating a fine white powder. Then I wipe with a tack cloth and then denatured alcohol. That dries almost instantly and I put on second coat. I sand with 400 next and repeat the previous wipe process. The final coat is applied and pretty much done after 3-4 hours. The result is a really nice even satin finish with no brush marks. I guess I'll keep doing it wrong until a problem shows up (never has)

Bob

[DerbyDad03]

Have you tried the same process with Wipe-On poly?

I hate painting, so I hate brushing. Wipe-On is so
easy and quick, but more expensive. I’m willing to
pay extra to keep my frustration levels low.

[Brian Welch]

Not sure, but just guessing a resolution was found in the past 14 years...
My suggestion, given enough time in advance, is to try it on a sample and see what happens...
Otherwise, I have found that the label on the PU can provides lots of information, given the proper strength of reading glasses...

[Bob Davis]

I hear you! I have not tried it with wipe-on. I came to terms with brushing just this year. I purchased various sized brushes from an artist supply. They are hugely better than anything you get in a buider's supply. I am using Varathane Ultimate Polyurethane water based satin finish. For cleanup, I just put the brush under the (gasp) kitchen sink faucet to wash off the bulk, then swish it around in a jar of water with a dash of dawn detergent, then rinse and set on the counter to dry. It takes less than a minute to clean a brush and its still soft and pliable when I use it again.

I decided to go back and read the label to see what I was doing "wrong".

1. Yep, dry time for first coat is 2 hours (test by lightly sanding to see if powder forms).
2. Apply second coat after first coat drys (NO sanding!)
3. Sand after second coat dries, IF the grain has raised.
4. Apply third coat BEFORE 24 hours has passed.

These instructions seem to go counter to some of the other processes expressed here. The varathane works so well for me that I have not been tempted to experiment with anything else. Leon is agitating about that monocoat thingy invented in Spain... Maybe. :-)

Bob

[Leon]

I think I will be resting the Rubio monocoat soon.

[DerbyDad03]

Resting? As in putting it away?

[Puckdropper]

DerbyDad03 <teama...@eznet.net> wrote in
news:c96f7cd8-5d27-49f0...@googlegroups.com:

> On Tuesday, October 26, 2021 at 1:16:50 PM UTC-4, Leon wrote:
>> I think I will be resting the Rubio monocoat soon.
>
> Resting? As in putting it away?
>

Sleeping? Didn't he mention something about a bed project a few weeks ago?
:-)

Puckdropper

[DerbyDad03]

I wonder if he has green sheets. ;-)

[Tbone123]

I think the question is, is it recommended to clean sand dust with mineral spirits before applying next coat?

[Bill]

On 5/31/2023 1:31 PM, Tbone123 wrote:
> I think the question is, is it recommended to clean sand dust with
> mineral spirits before applying next coat?
>

I sand lightly and clean. No mineral spirits are necessary.
At least, that's what I do, and it comes out very well.

[Markem618]

A tack cloth is probably better than mineral spirits, poor quality
solvents are likely to cause finishing issues.

[Scott Lurndal]

A tack cloth is useful to remove the sanding dust.

[Leon]

I use Festool sanders with a dust extraction so there is little to no
dust left on the surface, NOW.

Before dust extraction I simply used compressed air to remove the dust.
That worked very well.

Mineral spirits works well to remove anything missed by the sanders with
dust extraction.
Mineral Spirits gives you a preview of what the wood is going to look
like with varnish applied and evaporates relatively quickly and leaves
no residue nor does it raise the grain.