No need to sand between coats

[Steven Hsieh]

I never understood that people says

there is no need to sand between coats of spar varnish. As long
as no debries flies onto the door, you will be fine.

You will always have stuff flying on the door when the varnish is still drying

[Prashun Patel]

I've never heard that sanding between coats is a function of the TYPE of varnish or finish used.

I've heard (and I believe) it's a function of how thick the finish goes on, and how slowly it dries. The thinner the coat, the less likely the need to sand.

However, for me, the only rule to sanding is: if it's dry and it feels rough, sand.

[glenn bradley]

I'm no expert but, poly cans that I have read (sounds like a title for a movie) state that if you do not re-coat within a certain time (to allow burn in I assume) you wait 72 hours or so, sand to provide grip and then apply the next coat. Explanation from folks who know more than I is that poly doesn't stick to poly except by mechanical bond. So, a slick poly surface is not a good surface to apply poly to; peeling can result ;-)

As to dust ALWAYS getting onto the surface . . . . do you mean to say that you don't have a HEPA-clean, positive pressure containment facility within which to apply your finish??? Wahdda ya do, use the gara . . . er shop!?!

[Jim Rimmer]

Here's what Steve Mickley at Hardwood Lumber has to say (he is talking about a wipe-on finish). For more of his advice check out harwoodlumberandmoredotcom/articles

The other major misunderstanding in mastering the wipe-on technique for applying varnish is revealed by those who wipe on a coat, allow it to cure, sand, and wipe on another. This hold-over from the brushing mind set is unnecessarily slow, and ultimately will require more coats to achieve the same build. There is nothing sacred in the following schedule (there are many acceptable variations), but I have found it effective.

• Wipe on (as described above) the first coat of varnish. Then, go do something for 30-minutes or so before you come back and conduct the "pinkie test". The pinkie test consists of lightly touching the just applied varnish with the tip of your finger. If the finish is 'tack free' you are ready to wipe on another coat. If it is still sticky or tacky to the touch wait another 15-minutes and test again. Depending upon temperature and humidity the time between coats is usually 45-minutes to 1-hour, two-hours at the most. Clearly, since the varnish does not cure in this length of time you do not sand between coats. Wipe on three coats in succession allowing each to become "tack free" before the next coat is applied. Then, allow the varnish to 'cure' for 8 to 12-hours.
• When the first set of three coats has cured, lightly sand to remove any dust particles that may have settled in your finish. When sanding finish I prefer an open coat sandpaper such as 3M "Gold". Sand the first 'set' with P240 or P320 grade. If sanding produces a fine, dry, white powder the finish has cured sufficiently for sanding. If sanding causes the finish to 'rice' or ball-up on the paper, stop sanding and allow the varnish to cure longer. Clean away the sanding debris with a vacuum or with a shop towel lightly dampened with mineral spirits.
• Apply the second set of three coats following the same procedure used for the first set. After the varnish has cured lightly sand with P320 or P400 and clean the sanding debris as before. Check the finish carefully for any glitches you may have missed and correct any defects before you proceed to the third and final set of three wiped on coats.

[Scott Holmes]

I have talked with my buddy Steve many times. I know for a fact he is talking about NON POLY varnish... The reason I'm so sure... Steve only uses poly on floors and he would not do a wipe-on for a floor...

I use the same technique for wipe-on varnish and I too, never use poly on furniture.

Doing this with poly and you have a good chance of delamination between the ultra thin wipe-on coats... If you re-wipe as soon as it's dry-to-the-touch you may get good results... let it dry too long and look out.

[Steven Hsieh]

I'm no expert but, poly cans that I have read (sounds like a title for a movie) state that if you do not re-coat within a certain time (to allow burn in I assume) you wait 72 hours or so, sand to provide grip and then apply the next coat. Explanation from folks who know more than I is that poly doesn't stick to poly except by mechanical bond. So, a slick poly surface is not a good surface to apply poly to; peeling can result ;-)

As to dust ALWAYS getting onto the surface . . . . do you mean to say that you don't have a HEPA-clean, positive pressure containment facility within which to apply your finish??? Wahdda ya do, use the gara . . . er shop!?!

No, Im talking about a exterior door. But even a HEPA filter inside the shop does mean dust particles wont touch the surface.

[Prashun Patel]

I'll defer to Scott's advice, but it was my understanding that wiping on 3 three successive coats just after they pass the pinkie test actually results in a layer that fuses and cures as a single layer. Therefore, I'm not sure it matters what type of varnish you use. BUT, as always, I'm just parroting what I've read. I'll defer to any expert whose experience is to the contrary.

[Scott Holmes]

Prashun,

You are correct if you apply the next wipe-on coat as soon as it passes the pinkie test.

If you wait too long; poly will be the first varnish to have adhesion problems. With non-poly varnishes, e.g. Waterlox or Behlens there is no need to sand for adhesion for the first few days; by the 4th day it's a good idea to scuff sand.