Welcome to the forum, Back in the studio. As your work is mainly acrylic, I would rather ask this question in the acrylic forum.

Hi all –

I am new to this forum, and recently back in to painting, creating.
My mixed media pieces are large – 4 x 4′ at least and are dominantly acrylic with oil pastel accents and details on top. usually on canvas, these are heavily textured which causes finishing the work to be a little tedious. What is the best polyurethane finish considering rough texture and oil pastel being involved?? suggestions welcome.
Matte finish is my complete preference. thanks,

Since oil pastels really never dry / harden, whatever you put on there could possibly cause them to dissolve to some extent. Be sure you test it well before using it in a finished piece. Most likely you will need to spray it on in very light multiple layers.

If it were my work, I would probably go with a matte acrylic permanent varnish or medium, made for artists rather than a hardware store urethane finish, unless it is going to be displayed outdoors. Alternately, if you find a gloss finish that works well for sealing it, you could put a removable matte varnish as your final protective layer.

Personally, as a Picture Framer, the only safe way I can think of to protect that type of mixed media, without risk of damage, is to frame it under glass with a spacer between the painting and glass, even at that size.

I have only used it on acrylic paintings so far, but since it was originally formulated for oil paints, you could use GamVar over the whole piece.