I just took a pencil and passed it right over the lines as you can see here, I think it gives it just the right sheen, you just have to be careful not to handle it much....
For your valve covers, after you paint them black, preferrably Flat, you can use these pastes that I bought at Michael's Crafts Store. What you do is just dab a little bit on top of your workbench, put a little bit on your finger and after using your other hand as a pallete and taking some excess off just rub off the paste above the raised lines of your valve covers, you should be able to clearly see the lines without messing up the valve covers. You can also do this with a flat brush! At the end, if you want your valve covers with a bit of a semi gloss look you can always spray them at the end.