Hello Nalusurf and welcome to the Community.
For Versabond thinset, the recommended trowel size for a tile where the longest side of tile is less than 8" use 1/4" x 1/4" x 1/4" (6 x 6 x 6 mm) Square-Notch. The recommended type of trowel is usually listed on the adhesive package.
Thank you for your inquiry.
Charlotte.
Hey Nalusurf,
I'd like to add to what Char already stated.
ANYTIME you use 12" x 12" or smaller vertical (wall) tile, you always use a V-notched trowel rated for tile installations since it is made specifically for vertical surfaces to bond the mastic/mortar successfully.
Also, make sure the thin-set mortar you hand mix yourself is also rated for vertical surfaces. Most are not rated to apply on a vertical surface, save for the natural stone and large format tiles version of thin-set (Versabond & Customblend are not rated for walls). Others like those mentioned will not adhere properly and cause the tiles to slip right after applying it.
Hello Nalusurf,
Click HERE to see what some of my co-worker think about a similar situation. There are several opinions about trowel chooses. Below is another.
V-Notch Trowels
V-notch trowels are the preferred type for walls but they are not mandatory. The V-notch is superior because it applies slightly less mortar or adhesive onto the walls than a typical square or U-notched variant. This allows you to avoid over-smearing, which can lead to oozing from the joints between the tiles. If this happens, it makes grouting more difficult because you must first clean out all the excess thinset. There is still plenty of coverage for the tile to bond to, it’s just that the V-notched type has slightly less, just as much as a U-notched type applies slightly more. They are most common with small-bodied tiles up to 6 inches in size.
Because it is a back splash and you are pressing into place well, I think you will be alright.
Charlotte