How to put smooth coating on concrete windowsill - a grout?

[Maria]

I have a sculpted concrete windowsill (exterior) on my Victorian house, wh= ich had decayed and crumbled over the years. I got rid of all the paint and= rubbish on it, and rebuilt the profile using cheapo Wilko exterior filler,= smoothing it off with a wet trowel. It looks like a new sill now, except f= or a few patches where the filler is powdery (presumably because I overwet = it when smoothing off). What I would like to do is give it a final thin coat to fill in the rough p= arts and make it more stable - I did a quick search on the net, and saw an = article that suggested using a home made 'grout' for this purpose, which ca= n be put on thinly - to prevent the surface from crumbling due to its thinn= ess, they mixed it with something, but being a US site using US trade names= , I don't know what it is. Any ideas what I could mix with this filler to keep it smooth and adhesive = please? Thanks

[stuart noble]

had decayed and crumbled over the years. I got rid of all the paint and rubbish on it, and rebuilt the profile using cheapo Wilko exterior filler, smoothing it off with a wet trowel. It looks like a new sill now, except for a few patches where the filler is powdery (presumably because I overwet it when smoothing off).

parts and make it more stable - I did a quick search on the net, and saw an article that suggested using a home made 'grout' for this purpose, which can be put on thinly - to prevent the surface from crumbling due to its thinness, they mixed it with something, but being a US site using US trade names, I don't know what it is.

This is the stuff I'd use

easy to apply, but rock solid, even in very thin layers. Use the plastic applicator (or scraps of wood) instead of your wet trowel.

[Maria]

Excellent thanks - and should I use this stuff alone, or mix it with the mortar filler I've been usin?

[stuart noble]

mortar filler I've been usin?

Use it alone, and make sure surface is dust free and dry. As I said, not easy to use, so only mix up golf ball size bits at a time.