Replies

1. 
   BillBrennen | Sep 21, 2006 08:35pm | #1

   Yes, there is thinset that will stick to oil paint. The paint has to be fully cured and then roughed-up (abraded) to help the bond. Any highly modified thinset should do the job. Give the paint a few weeks to cure before tiling over it. If the paint adhesion is good to the wall, the tile should be fine.

   How big are the tiles? I ask because they will probably be frostproof, which means they will also be impermeable to water vapor. The oil paint is also a good vapor barrier, so the only way for the thinset's water to escape is at the tile edges. To do this soon, you have to have smallish tiles. Unlike straight Portland cement, highly modified thinsets will not fully cure until they dry out completely. Is this wall south-facing and getting sun?

   If the tiles will be large, a slightly modified thinset would be a better idea, as it is less dependent upon drying to attain its strength. Custom's Versabond is an example of this type of thinset.

   This time of year you may also have to begin thinking about possible freezing conditions. Maybe your climate there is mellow enough for that not to be an issue. A lot depends on how much heat is leaking out the block from inside the building, too. The thinset should not freeze for at least a week after application, more if larger tiles are used.

   Bill

1. 
   manoman | Sep 22, 2006 03:11am | #3

   The installation is a series of mosaics, the largest tile being less than a square foot. The tiles will be on all sides of the building (a school). We are in Victoria, BC, so no frost for a month or so, and little fear of frost problems, although the tiles are vitreous (frost proof). We have used Mapei products in the past, Keribond. I'm not sure the specs regarding how highly modified it is. Does that mean the amount of acrylic latex additive? We will mask the areas to be tiled so as not to wait for a cure on the new paint, clean it well with a pressure washer, rough it up with wire brush, wait a week to grout, to let the water evap.

1. 
   BillBrennen | Sep 22, 2006 08:31am | #4

   That all sounds real good. Mapei makes great mortars, talk to your supplier or the Mapei field rep for specific product recommendations. Some thinsets have polymers mixed into the dry powder, so they are modified even when mixed with pure water. Going over the old paint sounds like a great idea versus over new paint. Oils can take so long to cure hard in cool weather. Best of luck to you.Bill

1. 
   manoman | Sep 23, 2006 06:56am | #5

   What about using good ol' construction adhesive. It's a wall installation, no traffic load, no shower/water splashing to speak of. A good quality construction adhesive,,,the paint is sound, use c. adhesive like a mastic, notched trowel from a tub, back butter, wiggle it on there??? The stuff is waterproof, sticks to anything. (Masking went well, by the way...painters just sprayed right up to the tape/pattern...now we have layout lines... and touch up paint)

1. 
   BillBrennen | Sep 23, 2006 08:30am | #6

   No way on the construction adhesive. Thinset is cheaper and better. To get full coverage, which you want with tile, you would need huge amounts of mastic. Thinset can fill irregularities far better than mastic can. How smooth is the wall surface? If perfectly flat, then mastic could work, but why bother? Thinset is easier to clean up after, too.Bill

   Edited 9/23/2006 1:34 am by BillBrennen

2. 
   manoman | Sep 23, 2006 07:32pm | #7

   Yeah, that's what I thought. Someone mentioned it and it sparked my thought. The walls are stucco and concrete block. They are essentially flat, although there are the usual irregularities, mortar lines between the blocks, roughness of the stucco.
   Thanks for the help.

2. 
   sungod | Sep 22, 2006 02:56am | #2

   Get a power washer that's over 3000 psi and use the most narrow nozzle. If the washer doesn't blast the paint off, then thinset will work on it. If the block has been repainted because moisture made it peel, sandblast it.