I use them all the time. No matter what sealer, it WILL knock back the metal look by 10-15%. Stainless steal holds up the best. Titanium next. Graphite or Gun metal next, Burnished aluminum the least.
Use Tamiya weathering master pastels. (Or any other pastel) You can use water with some after you put them on (if your looking to simulate gooy oil leeks' of example). They will also diffuse more when you over coat them with gloss or dull coat. (which you need to do) They will look like you air brushed them. If you need it to be really shiny use Testers extreme gloss coat after your done. If you MUST use paint to weather ( acrylic or enamel) over coat your sealer with at least 4 to 5 coats. Also let your model cure for minimum 72 hours before you do any weathering.
A week is better if your not on a dead line.
If your NOT going to handle it at all ( going in a display case ) you can get away without using any sealer. But when you handle it, the Metalizer WILL come off on your fingers along with smudges or finger prints. (How would I know that?) sometimes you can polish them out, some time not.
Also I do not use the spung applicators that come with Tamiya weathering master. I use an old cut down skinny paint brush(s). Clip the ends so it is an 1/8 showing and grind it in to a paint palet to spread out the bristles as needed. That way you use the splayed out edge in crevasse etc. or use it to spread the pastels out.
Take an old model and test your technic first BEFORE you do any thing on the model your working on.. Sucks to have to go back and fix screw ups. (How would I know that?)
Hope this helps.
Best regards,
e