Hi everyone
its couple of years that I’m Facing the “White Spots” or “Artifacts” problem when using mentalray. afew days ago when I had a search around the net looking for a solution, I found out that almost there is no certain solution (or I dismissed to find) and solutions change depending on the lighting and shading tools used in scene. I’m not Mentalray pro but I decided to share some of my own Tricks around this issue based on my experiments not widely ,but Hope to be helpful for some (not many). here we are:
*** I Wish this topic to be posted in right place***
First of all, as you mostly know, the “white spots” Are Photon Error which appear when photons are in conflict by some objects or materials (of course when “photon emition” is enabled). So you gotta play around “indirect Illumination”
1- Spot the Wrong “Object” or “Shader/Material”
I Don’t know why, but in our Animation Project, “Raytrace” shader used in standard material shader slots, caused many of these photon emiting artifacts. After “Raytrace” shader, the most problem caused by “SSS Fast Skin” material (maybe more shader/materials cause this problem, but I didn’t face it until now). when you found out that which case are your problem, you can face it by these options :
A - Use Another Similar shader : I usually use Arch&Design instead of Raytrace shader, and render my skins by a gray standard material. forget about second , (no replacement for fast Skin).
B - Uncheck the “Generate Global illumination” in “object Properties” : it is another useful thing that exclude your bad shader/Mat to effect photon emiting proccess and mostly is a useful option:
C- Avoid your Objects to fall into each other : Again in SSS case, when we use Clothes on body, because of a tolerance that cloth process needs, sometimes the cloth fall into Body, or in any other case that An object are into another (bottle on the table maybe).specially when one of objects are reflective/refractive, So don’t let it happen. (delete your reflecting object if u needed to , it also reduce rendering time… forget this case also…)
2 - Refine your Global Setting :
A - use “Noise Filtering” as “extremely high” : this is another setting that reduce the color spots effect, depending on your scene you can use this option as “High” or “Extremely High” but before this, you’re going to need use “Final Gather” along “Global illumination” to light your scene, (or simply turn off all your lights , never mind…):
B - Enable and use a low amount for “Maximum Sampling radius” : can’t explain technically but it this option effects!(let you know when I find it out why) it works on a low value also using “noise Filtering” its own default : “Standard” .
C - Build A “Photon Map” under the “Caustics and Global Illumination” rollout : This case is almost a Silver bullet in the head of the artifacts, but depending on your scene, you may not be able to do so, But if you Do, just go to a frame that renders correctly (or initially hide your bad objects) , address a “photon map” on your hard disk and let MR to build a photon map from a correct rendered frame and let MR to read it in whole rendering proccess.
D - Turn off “Global Illumination” option, or Edit your output, frame by frame in a image editing Software , mostly photoshop : these options are also useful sometimes to save ur time and render the project for the boss as he expects
That’s one small step for MR, one giant leap for MR Atrists ! 
and don’t forget, please let me know if you have any solution for this that I didn’t find it in my whole digging in net.
Regards
Armin