How can I keep joints between form liners from showing in architectural concrete?
The usual method is to tape them. If nothing is done to seal the joints the concrete will bleed. When this happens the outline of the form liner will show on the wall as a dark line because the water/cement ratio has been altered at that point. This line cannot be eliminated by sandblasting because it goes at least 1/2 inch into the concrete. Taping, if properly done, keeps the joints sealed and prevents bleeding. Occasionally there has been the objection that even the tape thickness shows on fine work. For such applications it is possible to seal the joints successfully with a filler carefully worked into the joints. One suitable filler is a mixture of portland cement in lamb fat. This is fine-grained enough to fill narrow cracks and is water repellent.