Adhesives Part II:
Common Application Errors
[and how you can avoid them]
In our May 2011 issue, we addressed the difference between glues and adhesives. This issue we’ll help you tackle some of the most common application errors.
Spread & Surface Contact
If you apply an adhesive to a dirty surface, you stick to the dirt and hope the dirt is well stuck to the surface—so yes, you should clean the surface more than you usually do! Steel wool and solvent is essential for caulking and the quickest way to get off atmospheric grime. One exception is when a “clean” and flat residue is so well stuck (like old carpet adhesive) that the old can actually be considered a primer coat.
Clean and dry? Not always. Some adhesives, like polyurethanes, actually require a bit of moisture to properly cure. So we don’t want a wet surface that could dilute the adhesive, but silicones and polyurethanes and spray foams could actually profit from a slight misting of a dry surface before application.
Surface contact for the adhesive is critical. In woodworking we often apply glue to one side and then rub the two pieces together to assure full coverage of both sides. Large ceramic tiles can always profit from a very thin “back-buttering” of thin-set mortar on the tile itself before pressing into the troweled thin-set on the floor. This gives a mortar to mortar contact. When applying a bead of adhesive, like on the back of large panels, apply to one side only and then press it immediately into place, maybe slide it slightly to spread out the adhesive or pull it off and put it back (see solvent flashing below). This assures that the bead on the wall lines up perfectly with the bead on the panel. Underwater adhesives must be worked into the surface, essentially squeezing out all the water.
Skinning and Flashing
Skinning is not good if the contact to the surface is not yet made; we need it wet to the surface. Don’t get too far ahead of yourself in applying adhesives.
Solvent flashing is a process where we put the adhesive into place between two non-absorptive surfaces and then pull them apart to allow the solvents to partially evaporate, but not let the adhesive totally dry. Typically “flashing of the adhesive” is required behind vinyl tub surrounds over tiles to avoid trapping too much solvent, which in turn could burn into the vinyl.
Plasters and drywall compounds are actually one form of a gap-filling adhesive. When used between two panels where a lot of quick evaporation could cause problems, use powder mixed chemical setting compounds like DuraBond, which will cure before they slowly dry out.
The Interesting Case of Thinset Mortars
Latex or polymer modified thinset mortars have provided better tile adhesion to plywood substrates, but must never be used between tiles and a waterproof membrane; thinset morters will have trouble drying out and giving good bonding. Rather, you want to use unmodified thinset that cures before it dries. For example, when installing a Kerdi waterproof membrane in a shower or a Ditra uncoupling membrane on a floor or counter, use modified thinset between the plywood and the membrane and un-modified between the membrane and the tile.
A little tip on thin-set mortars: Each company makes a “good,” “better,” and “best” line of thinset price points, all of which satisfy the ANSI standards. Let’s admit that “good” translates to “minimally acceptable.” The actual difference between the minimum “good” and the more expensive “better” and “best” (the latter sometimes being referred to as “premium”), is they tend to be easier to spread, have increased tackiness at application and hold moisture longer for a better cure. Using these “premium” thinsets in jobs where maximum adhesion, even waterproofing, should be maintained, such as with shower membranes, not only assures good results but makes the application a breeze.
Long Term Degradation of Adhesives
Organic mastic paste can be used for tiles on walls, but if used in high water vapour areas the cured mastic itself can feed mould behind the tiles. Never consider the grout and even all tiles to be vapour proof! Use waterproof backings and inert thinset mortars to remove all food for mould in any constant moisture area.
You all know contact cement as a quick grab adhesive—let the solvent out then push the two pieces together. One problem is that you only get one chance at positioning the two pieces.
New Quick Grab or Power Grab adhesives have an extremely high tack but can be slid around before setting, specifically designed for mouldings and decorative add-ons with little or no nailing.
One tip for laminate edge trim pieces: Apply a first coat of contact cement to the edge of the particle board or plywood and let it dry overnight—really totally dry. Using water-based contact cement will work best because it is thinner and will soak in deeper. By letting a primer coat dry on the absorptive edge of the wood, the regular “coat both surfaces” technique the next day will not disappear into the wood but will stick extremely well to the primed edge—and you will never again go back to re-glue that edge trim piece. Even if using hot applied pre-glued banding tape, I prime the wood first.
Finally, good adhesive removers are showing up in the stores for that old carpet glue. They are labelled as being specifically for carpet glue. They are expensive and slow to act, but they work. Always cover with a plastic sheet to force the solvents into the glue rather than letting them escape without softening the glue.
The only silicone caulking remover that I can still find on the market, that won’t eat vinyl or fibreglass at the same time, is Re-Mov (www.RemovCanada.com). It does not soften the silicone but attacks the bond between the silicone and the surface; work it into the joint while pulling up the caulking like a rubber cord.
Montreal-based TV broadcaster, author, home renovation and tool expert
Jon Eakes provides a tool feature in each edition of Home BUILDER.