You can use Johnson's Klear/Future, which works quite well for a 'thin' mix, but I normally find that an enamel gloss varnish is better. It'll take a little longer to set, but not as long as varnish on its own, as the talc acts as a hardener. I'd suggest the latter for those joints. Apply it with a brush, into the joint, wiping off any excess with a tissue, then LEAVE IT! Once it's hard, then, if needed, a light sanding will do the business. Use the folded edge of fine wet dry, used wet, and sand gently. It shouldn't take much, and you'll find the joint is smooth and perfect. Use an old brush BTW, as it will be difficult to clean it thoroughly afterwards.
If unable to get/use varnish, then Wojtek's method is just as good, using light grey matt paint, preferably enamel, but acrylic should also work. I prefer varnish, as it tends to dry smoother, but the advantage with the paint is, you can see any imperfections or depressions immediately, and can re-coat as you go along. BUT, don't try to do it in one thick coat - it's better to have to repeat the exercise, than have a ridge to cope with! And, as with most modelling techniques, if in doubt, try it out! In other words, have a go on something unimportant first, if you are not sure what you are doing.
Cheers mate, good luck.
Terry.