These links explain how the chemical Tetrahydrofuran can be used to smooth and/or seal PLA 3D printed parts just like Acetone does with ABS:

https://www.youtube.com/watch?v=KJYaqaUr7iw

http://www.protoparadigm.com/news-updates/vapor-smoothing-and-polishing-pla-with-tetrahydrofuran-thf/

Puchase it here: http://www.amazon.com/s/?field-keywords=tetrahydrofuran

There are other postings about this method that suggest some experimentation is needed, but it looks promising for PLA parts.