I found that to be a problem with some panels I bought at Hobby Lobby. Way too slick. The first panel came out looking like a very watered down oil painting with a lot of white surface showing through. To get past that on the second panel I toned with a light colored acrylic and let it dry. Then painted oils over that and it came out beautifully. But, you’re right. A coat of gesso would achieve the same effect.