Will This Crack?

Mike Finley

On a recent job I had to do some drywall repair around a new fiberglass direct to stud shower surround. Because of some variables along one side there ended up being at gap between two pieces off drywall almost 1/2 inch wide. Using setting compound the gap was filled and covered with paper tape. Two more coats of setting compound smoothed the mess out and a final coat of all purpose finished the job and left it looking really nice, the time from 1st coat to final was probably less than 2 hours.

However I have my doubts as to the 1st gap filling layer of compound being really dry and wonder if this is going to end up cracking or am I worried about nothing?

Mike Finley

Damn were did all the sheet rockers go? Anybody?

PipeGuy

A guess only - the tape will preclude any minor cracks that might occur in the fill from mirroring through to the finish layers.

Atricaudatus

I could use some pointers on this issue as well. I am not a drywaller by trade (I'm a plumber/electrician) but I am renovating my own house. It was built in the early 40's and has experienced some minor settling. I have rocked and finished a couple of rooms so far and have had a couple of hairline cracks on seams.

I laid the mud in the seam, then embedding the tape (used paper on some, mesh on others) then a skim coat and let dry. Then a light sanding and a second coat with a wider knife, allow to dry, then another sanding and a top coat with a 16inch blade. I used all-purpose mud, maybe that was where I goofed.

Can anybody to tell me what I did wrong? I am getting ready to rock the second bathroom and don't want to repeat the same mistake.

Thanks, guys.

__________________
-Rhett
Licensed electrician and plumber

Mike Finley

From answers I have recieved on other forums, all purpose compounds dry by evaporation which leads to cracking when covering gaps. Setting compounds dry by chemical reaction and therefore don't shrink and lead to cracks forming when spanning gaps. Setting compounds also are much, much harder when dry which probably also has a lot to do with them not cracking in situations where all purpose compounds would.

The main benefit for me of setting compounds are the speed they offer, but they also offer the additional benefit of being able to span larger gaps without cracking between coats.

However if the problem is being caused by continuous settling I don't know how much the type of compound can help the situation.

mikesewell

MSG,
Setting compounds do shrink, and can crack when thick coats are used to span large gaps. Like concrete, setting compounds cure by a process known as hydration. And like concrete, the amount of shrinkage is a function of water content in the mix. Setting compounds and concrete do not "dry", they cure. "Dry" concrete weighs virtually the same as wet concrete. The liquid water in the mix becomes part of the crystal structure during hydration, and the water remains in the "dry" concrete.

Setting compounds have a different hardness than drying compounds because they consist of different substances. Drying compounds are mostly dolomite and water, with some polymers added. Lightweight compounds contain perlite. Setting compounds contain calcium sulfate. Either type will crack if you have differential settling. For cracks in "old work", sometimes an elastomer might be a better bet, for concrete we use polyurethane.

All the way,

__________________
Commercial construction

Sewell Construction

Atricaudatus

Thanks Mike!
That's the kind of thorough and thoughtful information I am rapidly coming to appreciate from the folks here.

By elastomer, do you mean latex caulk, or some product I can find in a bucket in the sheetrock aisle at Lowe's?

I am going to hang the rock soon, but still may sub out the taping and finishing. I seem to sand away more than I leave on the walls.

Not to drag this thread too far off topic (square me away, Teetor, if I am fouling up the boards) but how do I handle the seams where two boards butt together at the ends? The edges aren't tapered there to allow for the mud and tape. In the past I've just used ever wider knives to feather it out and sort of blend the hump in. Should I razor knife out a V notch or is that making the joint compound having to do more work and increasing the likelihood of a hairline crack?

Electrical work is a science, but mudding gypsum is an art; and I haven't mastered it fully yet.

This house was a serious "fixer-upper" when I bought it, but the price was right. In my spare tiime when I'm not out on the jobsite or doing paperwork, I am gradually redoing the whole place. Have added a full master bath, and am currently remodeling the hall bath. all new windows and Vynil siding is next, then the kitchen, and on and on. So, I will probably be bugging you guys for insight and opinions on a lot more things along the way.

Thanks again, and "Duty First!" :Thumbs:

"A little putty, a little paint... make it look like what it ain't."

__________________
-Rhett
Licensed electrician and plumber

Tom R

Never use spackling of any kind too fill gaps behind tape. Obviously, you want to try to avoid these gaps to begin with, - - but that being said, - - I usually would use something 'non-shrinking' that I have on hand, - - such as 'floor-leveler' or 'water-putty', - - keeping in mind these are both hard to sand, - - so make sure they're 'knifed' smooth while still wet, - - they'll both dry on their own in 20 minutes or so.

As far as 'mudding' end seams, - - like you say, - - it's somewhat of an art, - - but actually not all that difficult, - - they do sell a 'bow' trowel, you know, - - although it in itself can be somewhat tricky. Here's the procedure I use, - - skip the V-notch, - - that's not going to help you any, - - first 'mud-in' your 'bed' layer fairly thin, but consistent, - - then apply your tape normally, - - but 'knife' each side of the seam separately, - - let dry - - 2nd coat knife each side separately again, - - let dry, - - (the idea is to avoid creating 'build-up' in the middle), - - 3rd coat either use bow trowel or if you want to use a regular trowel, - - trowel 'across' the seam (at least 16" wide) to build up the middle slightly, - - let that dry and then 'pole-sand' parallel to the seam, - - avoiding too much removal in the center.