> Printing plexiglass can either be as an etching technique or a relief
> print or both together.
> If using the combined method use two different coloured oil based inks.
> My advice too, once you have the etching ink in the burr of the design
> wipe the surrounding areas very clean. With the second colour rolled up with relief ink, > use a BIG roller and I mean big roller that will cover the entire width of the plate in one
> roll or one circumference of the roller and clean the roller each time
> it is run across the design because the roller will pick up the colour
> of the etching ink and will leave dirty marks on the second half of the
> plate and consequently on to your print.....
The way to decrease the offset of the first color is to employ viscosity techniques. You'll want to make the intaglio ink extremely stiff and dry (with the aid of magnesium carbonate powder). The 'top coat' then should be very oily and loose - so add more plate oil than you would normally. You could add a third color with even more oil, if you'd like - but you're only making it more difficult on yourself when it comes time to clean up.
As Jan noted, you really want a wide diameter roller for this (and if doing more than two colors, you'll want a roller - from very soft to hard - for each ascending color. If the intaglio ink is very dry, the oiler ink should lay right over it with very little offset. Succesive layers will tend to blend more and offset onto each other - but create some interesting effects. In fact, some printmakers will purposely reverse the last two layers so that the drier roller pulls aways some of the oilier ink (rather psychedelic results).
One more bit of advice: when printing multiple colors like this, try to pick colors that are as far apart from each on the spectrum as possible. If the colors are too similar, they will blend together and you won't notice the difference - and all the extra work will have been wasted.