This was taken from www.fiberglassforums.com
, and was originally posted by Triple X
Several times before I have talked about the GREENSTAGE .
The Greenstage is a point after applying most body fillers where sanding is at its easiest. It is the time where the fillers exothermic reaction is almost complete and the filler is in a semi soft almost clay like state. Exothermic reaction is the chemical reaction that takes place when the elements in the catalyst and the filler react causing it to heat up, thus aiding with the curing process, and then cooling down allowing the curing to complete.
The Greenstage takes place at the very end of the process. It usually takes about 10 to 15 minutes on average after applying your filler(this will vary depending on the ratio of catayst added to your filler, the type of filler you are using, and the environmental conditions your are applying it in.
I have some pictures you can refer to along with some descriptions to go along that should help you to nail this tip/trick .. whatever you wanna call it..
either way .. it will make your sanding a lot smoother.. "pun intended"
STEP 1: Apply your filler. simple it seems, although made difficult easily by not taking care while spreading the filler to keep it as smooth and uniform as possible while minimizing spreader lines. This will GREATLY reduce the amount of sanding you have to do, period. Also, try adding just a little bit of resin to your filler before you add your hardener. Mix it up thoroughly and you will notice the filler has thinned just a little and will spread a tad bit easier.. Note: adding too much resin will cause your filler to be runny and no longer spreadable (so go easy on the resin), while adding just the right amount will allow the filler to mildly self -level, aiding in the removal of spreader lines..
also , when adding resin I personally will add just a couple cc's of mekp along with the cream hardener. Cream alone will work fine, but I like using a little of both.
STEP 2: This is where the routine changes a little. Normally you apply your filler and walk away letting the project cure completely.. not this time!..
I usually continue to spread the filler around until I can actually see and feel the filler begin to stiffen and . this allows me to remove any high spreader lines and also clues me in to when the exothermic reaction is really beginning to peak. You dont wanna walk away at this point and get a snack , or go take a crap(unless you really have to go bad, but you should of that of that before) because if you miss your brief opportunity, your screwed..
at this point ABOUT 10 to 15 minutes should have passed since you first applied your filler. now.. take a piece of sand paper between 36 to 40 grit but as high as 80 grit , and scrape it lightly against the curing filler. When its nearing the greenstage it will first roll off like clay and will look like this:
your almost there, but not quite..
keep lightly scraping the sandpaper every minute or so untill it begins to come of like parmesean cheese and looks exactly like this:
Now welcome to the GREENSTAGE!
STEP 3: At this point youll want to start sanding away. Have a few pieces of sandpaper ready to go in 36/40 grit, 80 grit and 120/150 grit. It helps if the sandpaper is back to back so it doesnt slip while your sanding during this stage You will begin with the 36/40grit , and get most of the spreader lines out, then continue with the 80 grit to remove the scratches from the 36/40 grit.
Keep in mind that the filler is much softer during this time and removes much faster than when fully cured. This is how its meant to be used to obtain the shape and or contour your shooting for. The main purpose here is to remove all of the spreader marks and achieve a smooth surface that follows the intended shape of your project WITHOUT removing too much filler from any area and comprimising that shape. You should sand in long motions to obtain uniformity and your sanded greenstage surface will look something like this when done correctly after 36 or 40 grit:
Notice the parmesean like texture of the filler after its sanded.. thats how it should look. Also keep in mind that every second that passes as your sanding , the filler continues to harden, and you will notice this as you sand since the parmesean cheese like texture will get thinner and dustier as you go. In other words, the faster you can go this , the better.
STEP 4: Lastly ,continue sanding your project with the 120/150 grit until its nice and smooth.. I dont have to say much about this as chances are you have been there before.. Just be sure to continue using long strokes and never sand too much in one place. After the 36/40 and 80 grit your surface should look like this:
At this point there are still scratches from the 80grit and maybe even some deep ones from the 36 or 40 grit , but nothing some detail sanding wont fix, not to mention if you plan to carpet or vinyl they usually are not visible anyway. If you do plan to paint then obviously sanding the scratches out and or applying a few layers of slick sand high build primer will fill em in nicely and can be sanded as smooth as you like to prep for paint.
Unfortunatly I do not have any picture from the later grits, but may add them later. The point here was the Greenstage Initial sanding and shaping , as opposed to finish sanding.
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