We have two Liquitex Slow-dri additives and two mediums. There are some three key differences: the gel and fluid mediums contain acrylic resins so you can add as much as you like to your color; the additives however do not contain resins and need to be handled with care – you can only add a small amount before you start to weaken the paint’s structure. The consistency is another difference – the gel medium and additive are perfect for thicker body paints like Heavy Body, and will thicken thinner paints. The fluid medium and additive are better for Soft Body and other more fluid paints, or for thinning. The final main difference is that the Slow-dri additives will NOT effect transparency levels of your color, while the mediums will.