You can, or just wash using your normal process for cleaning the car 1st. But, I recommend having the car clean 1st.
Why are you deconing the car?
1st make sure the car is totally cool and try your best to not do this outside. I usually do it in the garage out of the sun (make sure fans are blowing air out of garage). At 1st you might want to do 1/2 the car at a time to get the feel for the drying and working times. Work times will vary depending on what you're need for this system is in the 1st place. Heavy contaminants require longer work times ~ more product typically. 1st soap down the car and allow the product to bite into the surface/allowing it to work. Then, depending on how bad it is, reaggitate as needed. Rail dust deposites and water spots will generally need multiple aggitations. Again, make sure the product doesn't dry, especially on glass. Then rinse and move onto the next step after the surface is checked over. Hope this helps...