Expert: Daniel Humphrey - 5/23/2010

Question
QUESTION: Hi,

Is there any difference using joint compound on plywood from using wood filler?

ANSWER: Dear Shelley,

Joint compound isn't made to flex much or to be compressed under foot or get wet. Otherwise,  joint compound that sets up does act a lot the same as wood water putty.

I'm just guessing that you want to use it on a floor to fill the gaps and even out the bumps, in which case I would recommend not to use joint compound, but a quick-setting filler putty made for the purpose.  But if you are using it on a wall or shelving, joint compound might be fine.

Daniel

---------- FOLLOW-UP ----------

QUESTION: Hi,

Let me be more detailed so you can help me.  I'm building an art studio and exhibit space and using a/c plywood walls.  I need to fill the spaces between the boards and in the nail holes.  There is no heat in the building except for a portable heater and a/c that will run only when I am in the building.  It is a moist area near a river.  (Just giving you the general conditions).  Someone said that I can use joint compound..but my gut says to use water base wood filler.  Please advise.

By the way..what do you recommend for the filler putty for wide board floors (spaces between the boards)?

ANSWER: Dear Shelley,

You're a little bit avant-garde on this one! Those seams are likely to crack over time.

Knowing it might be temporary, for the walls, I would use a flexible caulk in the joints themselves, then paint the plywood with a primer and skim over the top with drywall mud before painting. I like to use a water-based caulk, or water-based acoustical sealant beneath the mud. Silicone will not work, as it repels any primer and mud.

For a better wall, apply sheetrock!

For the gaps in the flooring, I use Durham's Rock Hard Water Putty. If the gaps are very large, I mix in sawdust.

http://www.waterputty.com/pages/view.htm

Daniel


---------- FOLLOW-UP ----------

QUESTION: I'm confused.  Will the seams crack if I use wood filler?  How would I prevent that? I don't want to use drywall because taking nails in and out moving paintings around will destroy it.

What is the difference between caulk and joint compound?  What is water based acoustical sealant?  What mud are you referring to?

Sorry for all the questions but I'm not a handyman..just trying to prevenet a disaster.

Answer
Oh Shelley!

Sorry, I'm used to conversing with my workers and clients and hardware store persons. I don't get out much into the real public!

Caulk comes in a tube. Look for Polyseamseal, which is what I use. It's only drawback is that it shrinks quite a bit in curing. http://www.polyseamseal.com/allpurpose.shtml

For a caulk that shrinks less, look for siliconized acrylic painter's caulk, which has filler to keep it from shrinking so much. It makes a more presentable filler for painting over. I use DAP brand Alex Plus. http://www.dap.com/product_details.aspx?product_id=2

Acoustical sealant is a kind of caulk that is specifically made to be compatible with drywall mud. http://www.usg.com/sheetrock-acoustical-sealant.html

Drywall mud is another name for joint compound. Joint compound comes in two basic types: setting compound and non-setting joint compound. Setting compound sets up like plaster of Paris in about 90 minutes; regular joint compound stays soft until it air dries in 24 hours or so, depending on depth and humidity, and then gets harder from the carbon dioxide in the air. Both are brittle and will crack from a nail hole.

Durham's will not usually crack from a nail.

Plywood will contract and expand over time, causing the seams to show up in weeks or years, no matter what you do to fill the gaps. It sounds like that's okay, since the wall is going to be touched up frequently to patch nail holes and get fresh paint. For what you're doing, I'd use polyseamseal in the seams and Durham's the next day to fill low spots and make the plywood look good. After painting the first coat, I would use Alex Plus to fill anything you missed, then paint over. If you use a paint that happens to match the Alex Plus or the Polyseamseal, all the better, since you can fill anything after you painted by using caulk. Ployseamseal comes in a squeeze tube that is perfect for touchups.

Daniel