Some retailers also offer stackable trays in the same size ranges as flat files. These have open fronts, so that you just slide paper in and out. Stored this way paper is easily accessible and largely safe from light, though humidity and homesteading spiders will have free access. Once you've finished a painting, you can store it in a flat file or in a portfolio . But the portfolio is really the thing to protect the work during personal transport.
Portfolios come in three basic styles.
The plastic sleeves usually contain a sheet of black craft paper that act as a background and separator for art displayed on either side of the sleeve. Stored flat, the presentation portfolio provides a completely lightproof and acid free storage for your best artworks. Flat files are the best for long term storage in general, and for organizing your art into different categories. The other portfolios provide excellent storage, provided they are stacked flat, out of light, just like exposed sheets of paper. Large artwork stored in flat files or portfolios should be separated by tissue or glassine sheets to prevent the surface of a painting from being abraded by the back of the painting stacked on top of it.