Extender and Retarder are the same product, they both extend the drying time of your acrylic paint. As far as application, no method is better, it's more about which method works best for you.

I use mine 3 ways...I dip my brush into my extender and then my paint when using it in small areas. If you have trouble shading and highlighting these products can help with floating colors by putting the medium down on the area to be shaded/highlighted then adding your color, it makes shading and highlighting so much easier. For backgrounds I apply it to the surface before starting to paint to help blend colors, painting wet-on-wet (think Bob Ross).

I apply my backgrounds in several ways using large brushes, rollers, and sponges. It all depends on the size of the project and/or what I am trying to achieve. The best results I've had for sunset and blended sky backgrounds on projects is using an extender/retarder.
I used a large brush on the 2 sunset pieces pictured below and a roller on the seascape. You can see the difference in how the tool I used effects the look of the blending. On the seascape I used 3 colors and it's almost impossible to tell where one color ends and the next begins (love rollers). :-)

Kraftymom, thank you so much for those examples. That's exactly the instructional/visual information I was seeking. I am one of those who learns by reading/then by seeing. Your reply is going into my notebook of Tips and Techniques.

Great Questions and Great Answer!

Its nice to see Kraftymom back again and Welcome to Clarice, hope you come back often.

I have moved...sigh!...into a new area and have never used an extender and silly me didn't think about using one when I was making all of those similar but not the same backgrounds. How much easier it would have been to blend all of those colors. Your tip is now on my "remember to do" list.

Belle

I'm glad KM was able to help you in both words and those wonderful examples as well.

I was aware of extender, but have never taken advantage of it. After KM's explanation of how she uses it, I too will have to remember to give it a try.

Thanks for asking the question, we all learned from it. ;o)

Luvs

This helps a lot because I was wondering the same if extender and retarder was the same product. I'm wondering if they are the same then why not just have both names on one bottle instead of confusing us with two names. My biggest canvas I've painted is 26×20 but my sister just bought a huge one for me to paint the sky with clouds and the holy cross. My worry was not being able to blend the sky before it dries. Plus I'm having trouble with clouds. I've been watching videos on clouds and one paints them wet on wet using retarder in the sky. The other doesn't use it in the sky and uses extender in the clouds to blend. I'm so confused on which to use. What kind of roller did you use for your seascape?